sed: convert sed to use REG_STARTEND more explicitly.

sed: 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.

Thanks to Martijn van Duren and Ingo Schwarze for taking the time to
solve this in the least invasive way.

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