os/connection: Improve abstraction for launchd secure sockets

This changes away from hard-coding the /tmp/launch-* path to now
supporting a generic <absolute path to unix socket>[.<screen>]
format for $DISPLAY.

cf-libxcb: d978a4f69b30b630f28d07f1003cf290284d24d8

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
CC: Adam Jackson <ajax@kemper.freedesktop.org>