Acquire/release a POSIX file lock on repository archives.

- xbps_repo_open() accepts a third argument (bool) to acquire a POSIX file
lock on the repository archive.
- xbps_repo_close() accepts a second argument (bool) to release a POSIX file
lock on the repository archive.

This avoids the issue of multiple xbps-rindex(8) processes being blocked
even for different repositories on the same architecture, resulting in
unnecessary contention.

lib/rpool.c

@@ -107,7 +107,7 @@ xbps_rpool_foreach(struct xbps_handle *xhp,
 		xbps_array_get_cstring_nocopy(xhp->repositories, i, &repouri);
 		repo = xbps_rpool_get_repo(repouri);
 		if (!repo) {
-			repo = xbps_repo_open(xhp, repouri);
+			repo = xbps_repo_open(xhp, repouri, false);
 			SIMPLEQ_INSERT_TAIL(&rpool_queue, repo, entries);
 			xbps_dbg_printf(xhp, "[rpool] `%s' registered.\n", repouri);
 		}