Tailor \b{wb} for Perl

Tailor \b{wb} for Perl

The Unicode \b{wb} matches the boundary between space characters in a
span of them.  This is opposite of what \b does, and is counterintuitive
to Perl expectations.  This commit tailors \b{wb} to not split up spans
of white space.

I have submitted a request to Unicode to re-examine their algorithm, and
this has been assigned to a subcommittee to look at, but the result
won't be available until after 5.24 is done.  In any event, Unicode
encourages tailoring for local conditions.