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

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