BUG/MEDIUM: unix: completely unbind abstract sockets during a pause()

BUG/MEDIUM: unix: completely unbind abstract sockets during a pause()

Abstract namespace sockets ignore the shutdown() call and do not make
it possible to temporarily stop listening. The issue it causes is that
during a soft reload, the new process cannot bind, complaining that the
address is already in use.

This change registers a new pause() function for unix sockets and
completely unbinds the abstract ones since it's possible to rebind
them later. It requires the two previous patches as well as preceeding
fixes.

This fix should be backported into 1.5 since the issue apperas there.