posix: move server side of unix domain sockets out of inotify
We also turn the unix domain socket into a class, so that we have a
sensible place to hang its logically related attributes and behaviour.
We'll shortly want to reuse this in other code.
inotify: don't fall over just because of a dangling symlink
Previously, the inotify server failed to start if .hg/inotify.sock was
a symlink that pointed to a non-existent path. This behaviour does not
seem to make any sense.
Now, if we encounter a broken symlink, we unlink it and continue.