Add [[:blank:]] as suggested in

Add [[:blank:]] as suggested in

Subject: [ID 20000716.024] [=cc=] / [:blank:]
Message-Id: <200007170055.RAA23528@fummy.dsl.yahoo.com>

(the [=cc=] has already been taken care of by #6439
 so the whole bug report can be closed)

and make [[:space:]] to be equivalent to isspace(3)
(as opposed to \s, which is isSPACE()).  The difference
is that now [[:space:]] matches the mythical vertical tab,
while \s doesn't.

p4raw-id: //depot/perl@6703