specify the three grab modes as values in grab_client.ggo

specify the three grab modes as values in grab_client.ggo

That makes gengetopt check that the argument to the --mode
option is one of sloppy, pedantic, or aggressive, so get rid of
E_INVALID_GRAB_MODE, the gc_modes array and of the error check in
grab_client.c.

Another nice side-effect is that also "s", "p", "a" work as
abreviations for the grab mode argument.

Reformat grab_client.ggo as we are at it.