sed(1): convert sed to use REG_STARTEND more explicitly.

sed(1): convert sed to use REG_STARTEND more explicitly.

Summarizing the findings in the OpenBSD list:

This solves a reproduceable issue with very recent Mesa where REG_NOTBOL
combined with a match at the begin of the string causes our regex library
to treat the word as not begin of word.

Bump __FreeBSD_version: JIC we hit the issue in recent Mesa ports.

PR:		209352, 209387 (exp-run)
Taken from:     openbsd-tech (Martijn van Duren)
MFC after:	1 month

git-svn-id: svn://svn.freebsd.org/base/stable/10@302228 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f