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/show-info-files.c

@@ -307,6 +307,31 @@ repo_show_pkg_info(struct xbps_handle *xhp,
 	return 0;
 }
 
+int
+repo_cat_file(struct xbps_handle *xhp, const char *pkg, const char *file)
+{
+	xbps_dictionary_t pkgd;
+	char *url, *buf;
+
+	pkgd = xbps_rpool_get_pkg(xhp, pkg);
+	if (pkgd == NULL)
+		return errno;
+
+	url = xbps_repository_pkg_path(xhp, pkgd);
+	if (url == NULL)
+		return EINVAL;
+
+	xbps_dbg_printf(xhp, "matched pkg at %s\n", url);
+	buf = xbps_binpkg_get_file(url, file);
+	free(url);
+	if (buf == NULL)
+		return ENOENT;
+
+	printf("%s", buf);
+	free(buf);
+	return 0;
+}
+
 int
 repo_show_pkg_files(struct xbps_handle *xhp, const char *pkg)
 {