Add unix:dir=/something addresses

Add unix:dir=/something addresses

These are like unix:tmpdir=/something, except that the resulting
socket is always path-based, never abstract.

This is desirable for two reasons:

* If a Linux container manager wants to expose a path-based socket
  into the container, it can do so by bind-mounting it in the
  container's filesystem namespace. That cannot work for abstract
  sockets because they are not files.

* Conversely, if a Linux container manager does not want to expose
  a path-based socket in the container, it can avoid bind-mounting it,
  or bind-mount some harmless object like /dev/null over it.
  That cannot work for abstract sockets because access to abstract
  sockets is part of the network namespace, which is all-or-nothing.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=101567