perlop: Update text to reflect code changes

The warning message is no longer misleading, so no need to point out
anything about it.
perlop: Nits
perlop: Update to reflect 5.20 changes

Some deprecations have become fatal in v5.20
utf8.c: Use slightly more efficient macro

Lowercasing a Latin-1 range character results in a latin-1 range
character, so we can use the more restrictive macros that is slightly
more efficient than the general ones.  (This difference only is
applicable on EBCDIC platforms, as the macros all expand to nothing on
ASCII ones.