Fix compilation of initend for gcc 7

This is a trickier situation.

The original message:

```
initend.c:423:10: error: ‘%s’ directive output may be truncated writing 15 bytes into a region of size between 1 and 512 [-Werror=format-truncation=]
       "%s%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
          ^~
initend.c:422:3: note: ‘snprintf’ output between 16 and 527 bytes into a destination of size 512
   snprintf(xhp->confdir, sizeof(xhp->confdir),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       "%s%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       XBPS_SYSCONF_PATH);
       ~~~~~~~~~~~~~~~~~~
initend.c:429:7: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation ]
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "", buf);
       ^~~~~~~
initend.c:428:3: note: ‘snprintf’ output 2 or more bytes (assuming 513) into a destination of size 512
   snprintf(xhp->confdir, sizeof(xhp->confdir),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "", buf);
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
initend.c:434:9: error: ‘%s’ directive output may be truncated writing 17 bytes into a region of size between 1 and 512 [-Werror=format-truncation=]
      "%s%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
         ^~
initend.c:433:2: note: ‘snprintf’ output between 18 and 529 bytes into a destination of size 512
  snprintf(sysconfdir, sizeof(sysconfdir),
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      "%s%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      XBPS_SYSDEFCONF_PATH);
      ~~~~~~~~~~~~~~~~~~~~~
initend.c:455:11: error: ‘%s’ directive output may be truncated writing 14 bytes into a region of size between 0 and 511 [-Werror=format-truncation=]
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
           ^~
initend.c:454:3: note: ‘snprintf’ output between 16 and 527 bytes into a destination of size 512
   snprintf(xhp->cachedir, sizeof(xhp->cachedir),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       XBPS_CACHE_PATH);
       ~~~~~~~~~~~~~~~~
initend.c:461:7: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation ]
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "", buf);
       ^~~~~~~
initend.c:460:3: note: ‘snprintf’ output 2 or more bytes (assuming 513) into a destination of size 512
   snprintf(xhp->cachedir, sizeof(xhp->cachedir),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "", buf);
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
initend.c:467:11: error: ‘%s’ directive output may be truncated writing 12 bytes into a region of size between 0 and 511 [-Werror=format-truncation=]
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
           ^~
initend.c:466:3: note: ‘snprintf’ output between 14 and 525 bytes into a destination of size 512
   snprintf(xhp->metadir, sizeof(xhp->metadir),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "",
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       XBPS_META_PATH);
       ~~~~~~~~~~~~~~~
initend.c:473:7: error: ‘snprintf’ output may be truncated before the last format character [-Werror=format-truncation ]
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "", buf);
       ^~~~~~~
initend.c:472:3: note: ‘snprintf’ output 2 or more bytes (assuming 513) into a destination of size 512
   snprintf(xhp->metadir, sizeof(xhp->metadir),
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
       "%s/%s", strcmp(xhp->rootdir, "/") ? xhp->rootdir : "", buf);
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
```

It's basically warning about dangerous operations on strings. And as
far as I could tell, is a valid warning and not a false alarm!

This fix makes the concept of `XBPS_MAXPATH` lose a little bit of sense
as now it doesn't necessarily represent the max size of the paths used
by xbps, but instead the max allowed size of the path configured.

I think this change is ok, but I wasn't able to find any reference to
why it was chosen to be 512. POSIX mandates at least 256, so I'm not
breaking anything that wasn't broken already, and Linux seems to have
a maximum size of 4096, which is pretty safe.

Therefore, this changes should be harmless. I think.

include/xbps.h.in

@@ -592,7 +592,7 @@ struct xbps_handle {
 	 *
 	 * Full path to the xbps configuration directory.
 	 */
-	char confdir[XBPS_MAXPATH];
+	char confdir[XBPS_MAXPATH+sizeof(XBPS_SYSCONF_PATH)];
 	/**
 	 * @var rootdir
 	 *
@@ -606,14 +606,14 @@ struct xbps_handle {
 	 * Cache directory to store downloaded binary packages.
 	 * If unset, defaults to \a XBPS_CACHE_PATH (relative to rootdir).
 	 */
-	char cachedir[XBPS_MAXPATH];
+	char cachedir[XBPS_MAXPATH+sizeof(XBPS_CACHE_PATH)];
 	/**
 	 * @var metadir
 	 *
 	 * Metadata directory for all operations in XBPS.
 	 * If unset, defaults to \a XBPS_CACHE_PATH (relative to rootdir).
 	 */
-	char metadir[XBPS_MAXPATH];
+	char metadir[XBPS_MAXPATH+sizeof(XBPS_META_PATH)];
 	/**
 	 * @var native_arch
 	 *

lib/initend.c

@@ -397,7 +397,7 @@ int
 xbps_init(struct xbps_handle *xhp)
 {
 	struct utsname un;
-	char cwd[PATH_MAX-1], sysconfdir[XBPS_MAXPATH], *buf;
+	char cwd[PATH_MAX-1], sysconfdir[XBPS_MAXPATH+sizeof(XBPS_SYSDEFCONF_PATH)], *buf;
 	const char *repodir, *native_arch;
 	int rv;