libtt: if local hostname lookup fails, default to localhost

libtt: if local hostname lookup fails, default to localhost

A recurring problem, mainly on the BSD's, and also on some Linux
installations, is a failure for ttsession to start and any clients
trying to attach to it failing due to having a hostname that is not
associated with an IP address.

This is due to code which looks up the hostname, and if it does not
have a valid host record, then TT just fails.

This has required those users to add an alias for their host name in
their /etc/hosts file.

With this commit, this should no longer be necessary.  Now, if
_XGethostbyname() fails when looking up the local name, a default of
"localhost" is used, which should always exist.

This was run tested on Linux and FreeBSD 11.1.  It was compile tested
on OpenBSD 6.2.