Release version 1.8.0.

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).