test.pl: Add fresh_perl_unlike()
utf8.h: Use correct UTF-8 downgradeable definition

Previously, the macro changed by this commit would accept overlong
sequences.

The committer changed the original patch to swap a mask instead of a
test, in keeping with the prior version of the code; and to include
EBCDIC changes.
utf8.h: Use correct definition of start byte

The previous definition allowed for (illegal) overlongs
utf8.c: Use macros instead of if..else.. sequence

There are two existing macros that do the job that this longish sequence
does.  One, UTF8SKIP(), does an array lookup and is very likely to be in
the machine's cache as it is used ubiquitously when processing UTF-8.
The other is a simple test and shift.  These simplify the code and
should speed things up as well.
utf8.c: Clarify pod
utf8n_to_uvuni: Avoid reading outside of buffer

Prior to this patch, if the first byte of a UTF-8 sequence indicated
that the sequence occupied n bytes, but the input parameters indicated
that fewer were available, all n were attempted to be read
utf8n_to_uvuni(): Move checking for >32 bit code points

This just moves the code that does the checking if the code point is
non-portable to 32-bit machines to later.  These code points aren't
representable at all on EBCDIC platforms.

This will have the effect that on platforms where these aren't
representable, the error returned will be overflow instead of these,
but the moving fixes the problem where the first byte is garbled, and
the input really isn't such a large code point.  Prior to this patch,
the first byte is treated as gospel, and the intervening code points
aren't examined, leaving the pointer to the next input byte incorrectly
advanced too far.
utf8.c: refactor utf8n_to_uvuni()

The prior version had a number of issues, some of which have been taken
care of in previous commits; and some are left to do.

The goal when presented with malformed input is to consume as few bytes
as possible so as to position the input for the next try to the first
possible byte that could be the beginning of a character.  We don't want
to consume too few bytes, so that the next call has us thinking that
what is the middle of a character is really the beginning; nor do we
want to consume too many, so as to skip valid input characters.
The previous code could do both of these in various circumstances.

In some cases it believed that the first byte in a character is correct,
and skipped looking at the rest of the bytes in the sequence.  This is
wrong when just that first byte is garbled.  We have to look at all
bytes in the expected sequence to make sure it hasn't been prematurely
terminated.

Likewise when we get an overflow: we have to keep looking at each byte
in the alleged sequence.  It may be that the initial byte was garbled to
give us an apparent large number, but the actual sequence is shorter
than expected, and there really wouldn't have been an overflow.  We
want to position the pointer for the next call to be the beginning of
the next potentially good character.

This fixes a long-standing TODO from an externally supplied utf8 decode
test suite.
utf8n_to_uvuni(): Return REPLACEMENT not garbage

Given malformed input with no warnings, this function used to return
whatever it had computed so far.  But this is really invalid garbage.
Return the REPLACEMENT CHARACTER instead.
perlapi: Update for changes in utf8 decoding