bsdgrep(1): create rgrep link

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

A follow-up to r316473.

Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: emaste (earlier version), cem
Differential Revision: https://reviews.freebsd.org/D10109