enter-chroot: If XAUTHORITY is not set, make a guess

Useful when enter-chroot is called from udev/upstart, see #929.
Merge pull request #942 from drinkcat/enter-chroot-guess-xauth

enter-chroot: If XAUTHORITY is not set, make a guess
Fix licensing and update contributor instructions
Add vtlock command for tests to ensure graphics exclusivity
Don't explicitly include core.
Don't override the log fd
Add xephyr and xorg tests.
Fix exit inside tests.
Tweak test timing; unmount-chroot is still an issue.
Fix xorg/xephyr test command
Merge pull request #1 from dnschneid/x11test

X11test