BUG/MINOR: unix: better catch situations where the unix socket path length is close...

BUG/MINOR: unix: better catch situations where the unix socket path length is close to the limit

We do have some checks for the UNIX socket path length to validate the
full pathname of a unix socket but the pathname extension is only taken
into account when using a bind_prefix. The second check only matches
against MAXPATHLEN. So this means that path names between 98 and 108
might successfully parse but fail to bind. Let's adjust the check in
the address parser and refine the error checking at the bind() step.

This addresses bug #493.