gpm: Add runit service.

The run file could be configurable for more devices, but that comes later. Also, this is somewhat hacky since it relies on a bit of shell scripting to kill gpm (namely, the trapping).

It'd be much cleaner if the [verbosity patch from the mailing list](http://lists.linux.it/pipermail/gpm/2013-July/001134.html) was applied.