Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Build and install a BSD licensed grep.
If WITH_BSD_GREP is not set, it will be 'bsdgrep' and GNUgrep will be
'[ef]grep'. Otherwise, BSD-grep will be the grep family, and GNUgrep
will be 'gnugrep'.

Discussed with: brooks

- Change default grep back to GNU version. BSD grep can be built with the
WITH_BSD_GREP knob.
- Bump __FreeBSD_version

Requested by: dougb
Approved by: delphij (mentor)