croutonxinitrc-wrapper: Make sure children do not ignore signals

Creating a child resets signal handlers, but leaves ignored signals
unchanged (see "man 2 sigaction" for details): replace "" with a
no-op handler "true".

Test:
 - Logout in LXDE works (fixes #851)
 - Ctrl-C in crosh, or sending SIGTERM to xinit both work (exit
   commands are run).
Merge pull request #859 from drinkcat/xinitrc-fixtrap

croutonxinitrc-wrapper: Make sure children do not ignore signals
umount-chroot: Call umount one mount point at a time

umount, called with multiple parameters, may return 0 even if many
mount points cannot be unmounted, due to an overflow in its exit
code.

We fix this by passing `-L 1` to `xargs`.

Finally, we sort mount points in inverse alphabetic order, to
minimize the number of loops required.
unmount-chroot: -n 1 is good enough
Merge pull request #876 from drinkcat/fix-unmount

umount-chroot: Call umount one mount point at a time
croutonxinitrc-wrapper: Fix xbindkeys kill with dash

`wait "$!"` returns a >0 exit code when interrupted by a signal.
bash still executes the signal handler, then quits (-e), while
dash bails out immediately, leaving xbindkeys with PID 1 as parent.
Merge pull request #879 from drinkcat/fix-xbindkeys-kill

croutonxinitrc-wrapper: Fix xbindkeys kill with dash
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
test/run.sh: Ignore backup copies of tests

Some editors create a backup copy of the file, ending with a tilde:
ignore those files.
test/35-xorg: Dump OpenGL+dri information

Actual test will come later