libc: regex: partial revert of r368358

Part of the libregex functionality leaked into the tests it shares with
the standard regex(3). Introduce a P flag to set the REG_POSIX cflag to
indicate that l

libc: regex: partial revert of r368358

Part of the libregex functionality leaked into the tests it shares with
the standard regex(3). Introduce a P flag to set the REG_POSIX cflag to
indicate that libc regex should effectively do nothing while libregex should
specifically run it in non-extended mode.

This unbreaks the libc/regex test run.

Reported by: Jenkins

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libregex: implement \b and \B (word boundary, not word boundary)

This is the last of the needed GNU expressions before we can unleash bsdgrep
by default. \b is effectively an agnostic equivalent of

libregex: implement \b and \B (word boundary, not word boundary)

This is the last of the needed GNU expressions before we can unleash bsdgrep
by default. \b is effectively an agnostic equivalent of \< and \>, while
\B will match every space that isn't making a transition from
nonchar -> char or char -> nonchar.

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