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.

include/xbps.h.in

@@ -1269,6 +1269,8 @@ struct xbps_repo {
 	bool is_signed;
 };
 
+void xbps_rpool_release(struct xbps_handle *xhp);
+
 /**
  * Synchronizes repository data for all remote repositories
  * as specified in the configuration file or if \a uri argument is