bsdgrep: treat rgrep as grep -r and install rgrep symlink

MFC r316473: bsdgrep: treat rgrep as grep -r

MFC r316484: bsdgrep(1): create rgrep link

Create a convenience rgrep link for bsdgrep(1) that observes 'grep -r'
behavior.

A follow-up to r316473.

Approved by: emaste (mentor)