bsdgrep: Use implied working directory for -r if no directories are passed

MFC r317050: 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.

MFC r317300 (ngie): Only expect :grep_r_implied to pass with bsdgrep(1)

The test fails with gnu grep from base and ports.

MFC r319002 (ngie): :rgrep : use atf-check to check the exit code/save the
output of grep -r instead of calling grep -r without it, and saving the
output to a file

This ensures that any errors thrown via grep -r are caught, not lost, and
uses existing atf-sh idioms for saving files.

PR: 216307
Approved by: emaste (mentor, blanket MFC)
Relnotes: yes