Add systemd .service file for redshift

Add a systemd user .service file to allow users to run redshift as a
daemon using systemd. Also adds a new configure option,
`--with-systemduserunitdir`, which gives the install path.

If set to `no`, systemd support is disabled.
If not set, we use `pkg-config` to find the path.
Add systemd user unit file for redshift-gtk
Merge branch 'systemd-service'

Add systemd user unit files for redshift and redshift-gtk.
Fixes #35, fixes #50.