bus: touch() the AF_UNIX sockets we listen() on after the fact

bus: touch() the AF_UNIX sockets we listen() on after the fact

We'd like to use inotify to get notified when AF_UNIX sockets become
connectable. That happens at the moment of listen(), but this is doesn't
necessarily create in a watchable inotify event. Hence, let's synthesize
one whenever we generically create a socket, or when we know we created
it for a D-Bus server.

Ideally we wouldn't have to do this, and the kernel would generate an
event anyway for this. Doing this explicitly isn't too bad however, as
the event is still nicely associated with the AF_UNIX socket node, and
we generate all D-Bus sockets in our code hence it's safe.