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

share/man/man5/src.conf.5

@@ -732,9 +732,10 @@ and
 .It Va WITHOUT_GNU_GREP
 Set to not build GNU
 .Xr grep 1 .
-.It Va WITHOUT_GNU_GREP_COMPAT
-Set this option to omit the gnu extensions to grep from being included in
-BSD grep.
+.It Va WITH_GNU_GREP_COMPAT
+Set this option to include GNU extensions in
+.Xr bsdgrep 1
+by linking against libgnuregex.
 .It Va WITHOUT_GPIO
 Set to not build
 .Xr gpioctl 8

share/mk/src.opts.mk

@@ -99,7 +99,6 @@ __DEFAULT_YES_OPTIONS = \
     GCOV \
     GNU_DIFF \
     GNU_GREP \
-    GNU_GREP_COMPAT \
     GPIO \
     GPL_DTC \
     GROFF \
@@ -181,6 +180,7 @@ __DEFAULT_NO_OPTIONS = \
     BSD_GREP \
     CLANG_EXTRAS \
     DTRACE_TESTS \
+    GNU_GREP_COMPAT \
     HESIOD \
     LIBSOFT \
     NAND \

tools/build/options/WITH_GNU_GREP_COMPAT

@@ -0,0 +1,4 @@
+.\" $FreeBSD$
+Set this option to include GNU extensions in
+.Xr bsdgrep 1
+by linking against libgnuregex.