src.opts.mk: switch to bsdgrep as /usr/bin/grep

This has been years in the making, and we all knew it was bound to happen
some day. Switch to the BSDL grep implementation now that it's been a
little more thoroughly tested and theoretically supports all of the
extensions that gnugrep in base had with our libregex(3).

Folks shouldn't really notice much from this update; bsdgrep is slower than
gnugrep, but this is currently the price to pay for fewer bugs. Those
dissatisfied with the speed of grep and in need of a faster implementation
should check out what textproc/ripgrep and textproc/the_silver_searcher
can do for them.

I have some WIP to make bsdgrep faster, but do not consider it a blocker
when compared to the pros of switching now (aforementioned bugs, licensing).

PR:		228798 (exp-run)
PR:		128645, 156704, 166842, 166862, 180937, 193835, 201650
PR:		232565, 242308, 246000, 251081, 191086, 194397
Relnotes:	yes, please

share/mk/src.opts.mk

@@ -68,6 +68,7 @@ __DEFAULT_YES_OPTIONS = \
     BOOTPARAMD \
     BOOTPD \
     BSD_CPIO \
+    BSD_GREP \
     BSDINSTALL \
     BSNMP \
     BZIP2 \
@@ -203,7 +204,6 @@ __DEFAULT_YES_OPTIONS = \
 __DEFAULT_NO_OPTIONS = \
     BEARSSL \
     BHYVE_SNAPSHOT \
-    BSD_GREP \
     CLANG_EXTRAS \
     CLANG_FORMAT \
     DTRACE_TESTS \