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/

Change-Id: I9dc42c4105c35c42c6b195c880c70f4398852886