perlfunc: Update -B, -T descriptions

These descriptions were way out-of_date.
unicode_constants.h: Add definitions for ESC and VT

These will be used in future commits
pp_sys.c: Generalize -B, -T for EBCDIC, clean up

There was discussion about extending these to work for Latin1,
(http://nntp.perl.org/group/perl.perl5.porters/214950 and following)
but in the end it was decided that this is a heuristic, and perturbing
that might break things.  So this patch keeps the same criteria as
before, but refactors things so it isn't ASCII-platform-centric.

However, the version it replaces checked for utf8ness piece-meal,
leading to possibly incorrect results, which this fixes.  If a buffer
isn't entirely UTF-8, then it can't be UTF-8.  So this uses
is_utf8_string() instead of reimplementing the utf8ness checks, and does
it for the buffer as a whole.

And, there was an inapproptiate test for ALPHA under locale that this
removes.  The test for locale non-strange characters is that they be
either printable or space (things like \t are space but not printable).