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-query/ownedby.c

@@ -136,7 +136,7 @@ repo_match_cb(struct xbps_handle *xhp,
 
 	bfile = xbps_repository_pkg_path(xhp, obj);
 	assert(bfile);
-	filesd = xbps_get_pkg_plist_from_binpkg(bfile, "./files.plist");
+	filesd = xbps_binpkg_get_plist(bfile, "/files.plist");
 	if (filesd == NULL) {
 		xbps_dbg_printf(xhp, "%s: couldn't fetch files.plist from %s: %s\n",
 		    pkgver, bfile, strerror(errno));