Support for running Redshift outside X

Using Direct Rendering Manager, Redshift can now change gamma ramps for the monitors while running in Linux's TTY. This requires that the users is a member of the group `video` (as is always required when working with graphics without a display server.)

If Direct Rendering Manager is used to change the gamma ramps while an graphical session is active in the foreground (and X display is running on the active VT) Direct Rendering Manager will ignore the request and report that you do not have sufficient permissions. This rejection is ignored so nothing funny happens if the users opens a VT with a graphical session.

Changes to gamma ramps using Direct Rendering Manager are in effect on all VT:s with the exception on graphical ones because the X servers keeps tracks of gamma ramps for each display and applies the gamma ramps associated with a display when it is reactivated.

Possible improvements:
- Figure out whether Redshift is associated with an X display, and user DRM otherwise.