Support LAUNCHD code on Linux. Enable by default.

The launchd code allows $DISPLAY to be set to any path name, which allows placing the X connection socket in a more secure location in the filesystem than /tmp.

This is intended to help fix the security issue raised here:

• https://www.openwall.com/lists/oss-security/2020/11/09/3
• https://lists.x.org/archives/xorg-devel/2020-November/058626.html

Signed-off-by: Keith Packard keithp@keithp.com