Changes:
- authproto_pam: all pam_setcred.
- main: detect current Xss state at startup.
- main: wakeup on SIGUSR2.
- saver_mpv: perform playlist looping inside mpv.
- saver_multiplex: export screen saver index as XSCREENSAVER_SAVER_INDEX to
sub-savers. - XSECURELOCK_MULTIPLEX_BACKGROUND_COLOR: configurable background color.
- XSECURELOCK_SAVER_DELAY_MS: allow sleeping before mapping windows, for
smoother initialization of children. - XSECURELOCK_VIDEOS_FLAGS: flags for mpv (and also mplayer).