Change \p{} matching for above-Unicode code points

Change \p{} matching for above-Unicode code points

http://markmail.org/message/eod7ukhbbh5tnll4 is the beginning of the
thread that led to this commit.

This commit revises the handling of \p{} and \P{} to treat above-Unicode
code points as typical Unicode unassigned ones, and only output a
warning during matching when the answer is arguable under strict Unicode
rules (that is "matched" for \p{}, and "didn't match" for \P{}).  The
exception is if the warning category has been made fatal, then it tries
hard to always output the warning.  The definition of \p{All} is changed
to be qr/./s, and no warning is issued at all for matching it against
above-Unicode code points.