XRandR: Allow multiple but not all CRTCs to be redshifted

Previously only one CRTC could be set in the configuration file for
redshifting when XRandR mechanism was being used. That is fine for
a setup with two displays but breaks when three or more displays
are in use and one of those shouldn't be redshifted (e.g. two
computer displays and one TV connected to the computer).

The config value 'crtc' for method xrandr can now be entered as
comma separated list of multiple CRTCs. All CRTCs in the list will
be redshifted while all those not in the list will not be touched.

Signed-off-by: Lennart Sauerbeck <devel@lennart.sauerbeck.org>