agetty: use the "best" interface for \4 and \6

The issue file escape sequences \4 and \6 prints the host IP when no
interface is specified. That's useless on some virtual machines where
gethostname()+getaddrinfo() returns 127.0.0.1.

The seems better to print IP of the "best" interface (UP, RUNNING, non-LOOPBACK)
and use gethostname() as painful fallback only.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1090935
Signed-off-by: Karel Zak <kzak@redhat.com>