Add XRandR support.

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.