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).

Possibly interesting use: xprop -id `./get-compositor`.

Doing so is harmless as we're OverrideRedirect anyway, but may also help
a compositor ignore our window.

saver_multiplex: fix another stuckness issue made visible by 47a634fc, and add an automated test to detect it (and other screen resize issues) in the future.

Don't run tests with a Xinerama setup because Xephyr does it wrong (root window instantly resizes to the first screen, and Xrandr info is incomplete).

Set WM_NAME, WM_COMMAND, WM_CLASS on xsecurelock's windows for easier debugging and compositor configuration.

- Annotate some intentionally unused variables with (void).

- oclint found a bug in saver_multiplex that causes unnecessary screen
  saver restarts when receiving an XRandR event without a contentful
  configuration change.

This fixes:
- A potential bug in the "guessed monitor layout" code that may detect
  less monitors than correct (XRandR code path not affected).
- Unused values assigned to variables (annotated with a (void) cast
  where necessary).
- num_monitors variable redundantly checked for zero in cases where
  conditional compilation removes all code that would set it.

The configure macro had a small typo that made the default always check
- but luckily the Debian package isn't affected as it passes all desired
--with options anyway.

This prevents the input field from overflowing into neighboring
monitors.

This makes sure that the auth prompt always appears in the center of
every screen, and runs a separate screen saver on each one.

Further changes:

- Replaced an XSync by an XFlush, somewhat speeding up auth_pam_x11.
- Added a build dependency on XRandR to the Debian package.
- Global clang-format and IWYU.
- saver_xscreensaver now uses the parent process ID as random source
  instead of $RANDOM; this makes sure both monitors show the same screen
  saver.

This fixes cursor appearing on top of the screen lock in Cinnamon.

Revert "Explicit hide the cursor of the Composite Overlay Window." because this breaks the Cinnamon task bar.

This reverts commit e59e9575.

We seem to need this for Cinnamon but not for other desktops.

Fix a minor bug (uninitialized Pixmap data) in creating the empty cursor that might cause a black 1x1 point cursor.

The old code did not handle processes being terminated by signals right,
and basically waitpid() on them a second time, receiving ECHILD. This
didn't cause any real bugs, but was nevertheless incorrect.

This fixes artifacts that would come from deleting input after having
entered a password longer than the other text in the prompt.

We now require explicit opt-out to compile on systems without these being
installed; this prevents privacy issues from accidentally compiling with
XComposite support (which will let desktop notifications appear above
the screen lock).

Don't kill the saver_child's process group a second time when getting ECHILD and already having killed it.

This should not change anything noticeable - just a bit more consistent
that way.