Separate out adjustment methods from redshift.c into adjustments.h so that it can be accessed from elsewhere, and make the compile-time configurable

Signed-off-by: Mattias Andrée <maandree@operamail.com>
Use PATH_MAX from limits.h in config-ini, except under Windows

Signed-off-by: Mattias Andrée <maandree@operamail.com>