Add dummy gamma adjustment method

This method does not do anything but print the temperature to the terminal.
An external program implementing a color temperature adjustment scheme that
is not supported by redshift can use this method to determine the desired
color temperature. It is also useful for testing other parts of redshift on
systems where neither RandR nor VidMode works.
When in print mode do not start a gamma method

A gamma method is not needed in print mode
Constify gamma parameter to set_temperature() methods
Do not allow NULL keys when setting options

This should fix a bug where command line options for the "manual" location
provider cannot override the config file. To keep compatability with previous
versions the command line parser will still special case parsing "-l LAT:LON"
and set the correct options in "manual".
geoclue: Disable deprecated g_type_init() when glib >= 2.35