MFC r348503, r351769: bsdgrep nits

r348503:
grep: Move lone 'r'grep case into the adjacent switch

This 'r' case should have belonged to the switch in the first place, but
I had somehow missed the switch when initially adding the rgrep link. The
zgrep script later came along and faithfully left this case standing alone,
so we will now go ahead and join it.

Nearby comment also adjusted a tad bit for wording and style.

r351769:
bsdgrep(1): add some basic tests for some GNU Extension support

These will be expanded later as I come up with good test cases; for now,
these seem to be enough to trigger bugs in base gnugrep and expose missing
features in bsdgrep.