Fix get_fullhostname when src is already a FQDN and src/dst are the same

Several places in the code give get_fullhostname the same source and
destination buffers, meaning "update my buffer to be the full hostname".
If the source is already a FQDN, don't attempt to snprintf over itself.

This was broken by commit 770dfad7b2336a2ee508d05693737d08ddd9357f