bsdgrep: disable GNU_GREP_COMPAT by default

bsdgrep: disable GNU_GREP_COMPAT by default

The GNU extension bits in the base system are old, no longer faithful
to upstream, and surprising in some regards. Switch to documenting
WITH_GNU_GREP_COMPAT and default GNU_GREP_COMPAT to OFF in the name of
good behavior.

According to http://www.regular-expressions.info, GNU extensions:

-  Add missing quantifiers to BREs: \?, \+

-  Add branching to BREs: \|

-  Add backreferences (\1 through \9) to EREs

-  Add \w, \W, \s, and \S corresponding to :alnum:, [^[:alnum:]],
   :space:, and [^[:space:]] respectively

-  Add word boundaries and anchors:
   \b: word boundary
   \B: not word boundary
   \<: Strt of word
   \>: End of word
   \`: Start of subject string
    \': End of subject string

These extensions are still available in /usr/bin/grep by default today,
as it is still GNU grep.  As part of the bsdgrep migration plan these
extensions may be added to bsdgrep's regex support if necessary.

Submitted by:	Kyle Evans <kevans91 at ksu.edu>
Reviewed by:	cem
Differential Revision:	https://reviews.freebsd.org/D10114