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