Add '--help' to hybrid-detect

When looking at /etc/init/hybrid-gfx.conf or hybrid-detect one could maybe guess its use, but there is no description in the upstart job, no manpage, no 'hybrid-detect --help' or similar.

I propose adding an '--help' option to hybrid-detect with an output like this:

printf("Usage: %s\n", argv[0]);
printf("Detect if the available GPUs were changed (e.g. by BIOS settings) since the last run using %s.\n", FILENAME);
printf("In this case detect which GPU in a hybrid graphics configuration should be used.\n");