Fix pkg_set command for REPONAME attr.

Reponame attr should not set repo url, as repo url is a separate
package attribute.
Fix different errors in pkgdb logic.

* Remote repo is attached identified by its reponame (e.g. repo-something),
hence in rquery do not strip 'repo-' prefix.
* Oldsize variable if newer_than_local_pkg should be of int64_t type
and not char *, fix memory corruption thus.