resolv: Introduce struct resolv_conf with extended resolver state

This change provides additional resolver configuration state which
is not exposed through the _res ABI.  It reuses the existing
initstamp field in the supposedly-private part of _res.  Some effort
is undertaken to avoid memory safety issues introduced by applications
which directly patch the _res object.

With this commit, only the initstamp field is moved into struct
resolv_conf.  Additional members will be added later, eventually
migrating the entire resolver configuration.