If selected saver is "one" but the saver index can't be found, assume
"random-same" again like in the past.

This implements the distinction between "Random Screen Saver" (different
per screen) and "Same Random Saver" (same saver on all screens) in
xscreensaver-demo's settings.

Visibly here means that the cursor position is not updated, to make
clear to the user that these keys do nothing.

Related to #25.

Support Ctrl-U to clear the password entry line, and document behavior of some other screen lockers regarding this.

Leaving open how to handle Ctrl-A yet though.

Related to #25.

This does nothing in KWin though (works anyway), and it looks like other
compositors currently do not support this flag anyway. This makes it
however easier for compositors to fix incompatbilities by adding support
for the flag.

If this should happen, we'll just map it again. Yes, this is still bad,
as that means that after unlocking the screen the output may be still
black - but at least one can workaround that by forcing Compton to
redirect again (e.g. by alt-tabbing around). Better than revealing
screen content despite lock.

Explicitly XCloseDisplay before exit (makes sure our windows actually get properly destroyed, leaving black background and not the last password prompt on the display), and add a test case tool that breaks compton --background glx.

saver_xscreensaver: also parse 'mode' and 'selected' so one can better configure this using xscreensaver-demo

They're already mlock'd anyway, but this is still better e.g. against
physical attacks.

Helpers: remove bash dependency.

cppcheck: Fix unportable use of getpwuid() (PAM might call this too and overwrite our username buffer).