boinccmd: add new gpu_mode 'always_until_sigterm'

This new option to '--set_gpu_mode' sets the client's gpu_mode to 'always' and leaves boinccmd running until it catches 'SIGTERM', which sets the client's gpu_mode (back) to 'never' and terminates boinccmd.

I'm not sure whether this should really be merged into (upstream) 'master'. Its sole purpose it to provides a dummy GPU application to manage GPU usage of a running BOINC client from Condor.