XQuartz: Ensure we wait for the server thread to terminate

AKA: XQuartz 2.7.5 doesn't delete its /tmp/.X$d-lock

http://xquartz.macosforge.org/trac/ticket/823

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Merge remote-tracking branch 'jeremyhu/master'