Tweak the definition of the bit complement on UTF-8 data:

Tweak the definition of the bit complement on UTF-8 data:
if none of the characters in the string are > 0xff,
the result is a complemented byte string, not a (UTF-8)
char string.  Based on the summary in

Subject: Re: [ID 20000918.005] ~ on wide chars
Message-ID: <jSDD6gzkgi/T092yn@efn.org>

This should give us the maximum backward (pre-char string)
compatibility and utf8 compatibility.  The other alternative
would be to limit the bit complement to be always byte only,
taking the least significant byte of the chars.

p4raw-id: //depot/perl@7665