Improve xbps_end() and use it before exit(3)ing.

- Simplify xbps_repo_open::repo_get_dict().
- Use xbps_end() in the utils where necessary.
- Make xbps_end() call xbps_pkgdb_unlock() if necessary.
- Make xbps_end() release rpool resources.
- Make xbps_end() release resources from xbps_handle.
- Fixed 90% of reported leaks (still reachable at exit) from valgrind.

That was to silence valgrind's memcheck with --leak-check=full.

lib/initend.c

@@ -502,6 +502,7 @@ xbps_end(struct xbps_handle *xhp)
 	assert(xhp);
 
 	xbps_pkgdb_release(xhp);
+	xbps_rpool_release(xhp);
 	xbps_fetch_unset_cache_connection();
 }