MFC r300683:
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)

MFC r300378:
libc/regex: fix two buffer underruns.

Fix some rather complex regex issues found on OpenBSD as part of some
ongoing work to fix a sed(1) bug.

Curiously the OpenBSD tests don't trigger segfaults on FreeBSD but the
bugs were confirmed by running a port of FreeBSD's regex under OpenBSD's
malloc. Huge thanks to Ingo for confirming the behavior.

Obtained from: OpenBSD (CVS 1.20, 1.21)

MFC r298521;
regex: prevent two improbable signed integer overflows.

In matcher() we used an integer to index nsub of type size_t.
In print() we used an integer to index nstates of type sopno,
typedef'd long.
In both cases the indexes never take negative values.

Match the types to avoid any error.

MFC r265202:
Remove some unreachable breaks in regex.

This is based on a much bigger cleanup done in Illumos.

Reference:
https://www.illumos.org/issues/2077

MFC r265202:
Remove some unreachable breaks in regex.

This is based on a much bigger cleanup done in Illumos.

Reference:
https://www.illumos.org/issues/2077