This commit does two related things, from opposite ends of the spectrum:
* Sets the sticky bit on pid files and unix sockets to avoid tmpwatch
deleting them in cleanup scenarios
* Falls back to looking at the changed time if the filesystem doesn't
support reporting creation time when wezterm does its own liveness
and cleanup checks for unix domain sockets in the runtime dir
* Allow any wezterm instance to perform that cleanup
refs: #1601
When running the GUI, we generate a unix domain socket path for
the current process and start up a mux server for that path.
This allows `wezterm cli list` and `wezterm cli split-pane` to
work implicitly inside the GUI session.
When started in this way, the mux server is not persistent;
when the GUI process is terminated, all of its windows, tabs
and panes are terminated.
refs: https://github.com/wez/wezterm/issues/230
