libc: regexec(3) adjustment.

Change the behavior of when REG_STARTEND is combined with REG_NOTBOL.

From the original posting[1]:

"Enable the assumption that pmatch[0].rm_so is a continuation offset
to a string and allows us to do a proper assessment of the character
in regards to it's word position ('^' or '\<'), without risking going
into unallocated memory."

This change makes us similar to how glibc handles REG_STARTEND |
REG_NOTBOL, and is closely related to a soon-to-land fix to sed.

Special thanks to Martijn van Duren and Ingo Schwarze for working
out some consistent behaviour.

Differential Revision: https://reviews.freebsd.org/D6257
Taken from: openbsd-tech 2016-05-24 [1] (Martijn van Duren)
Relnotes: yes
MFC after: 1 month