bsdgrep: for -r, use the working directory if none specified

This is more sensible than the previous behaviour of grepping stdin,
and matches newer GNU grep behaviour.

PR: 216307
Submitted by: Kyle Evans <kevans91 at ksu.edu>
Reviewed by: cem, emaste, ngie
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/