x11/xscreensaver: Update to 6.05

Changelog:

• X11: Cope with dumb DPMS settings that existed pre-startup.
• X11: Silence new Perl warnings from xscreensaver-getimage-file.
• X11: Fix sonar pthreads crash on recent Pi systems.
• X11: Removed dependence on gdk-pixbuf-xlib-2.0.
• X11: GTK 3 is now required.

Approved by: x11 (manu, maintainer), cy, tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D37461