xbps-query(8: added --cat=FILE mode support.

- This mode prints to stdout the matching FILE stored in a binary package.
- ABI break: renamed xbps_get_pkg_plist_from_binpkg() xbps_binpkg_get_plist().
- Added xbps_binpkg_get_file() as a generic way to get pkg file contents.
- Removed useless comments from xbps_api_impl.h.

bin/xbps-rindex/index-add.c

@@ -80,7 +80,7 @@ index_add(struct xbps_handle *xhp, int args, int argmax, char **argv, bool force
 		/*
 		 * Read metadata props plist dictionary from binary package.
 		 */
-		binpkgd = xbps_get_pkg_plist_from_binpkg(pkg, "./props.plist");
+		binpkgd = xbps_binpkg_get_plist(pkg, "/props.plist");
 		if (binpkgd == NULL) {
 			fprintf(stderr, "index: failed to read %s metadata for "
 			    "`%s', skipping!\n", XBPS_PKGPROPS, pkg);