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.
This is primarily for improved discovery. Some users report that they didn't know about redshift-gtk, but had it been available on tab completion they would have noticed. Also, I think it is in general good practice that closely related programs have the same prefix.
Jon Lund Steffensen