Using strncpy meant that if listenaddress were ever >=
sizeof(sockaddr_un.sun_path), we would fail to nul-terminate
sun_path.  This isn't a big deal: we never read sun_path, and the
kernel is smart enough to reject the sockaddr_un if it isn't
nul-terminated.  Nonetheless, it's a dumb failure mode.  Instead, we
should reject addresses that don't fit in sockaddr_un.sun_path.

Coverity found this; it's CID 428.  Bugfix on 0.2.0.3-alpha.