"\cX" where X is non-printable is now fatal

It had been deprecated, scheduled to be removed in 5.22.
dquote_static.c: Clarify code

We output the message when there is a printable ASCII character, so use
that as the test instead of complementing the test for a control.  This
makes the code somewhat clearer.
Unicode::UCD.pm: Pod clarifications and nits
regexec.c: Fix some EBCDIC problems

We were testing for UTF-8 invariant, when we should have been testing
for ASCII.  This is a problem only on EBCDIC platforms, where they mean
two different sets of code points.
handy.h: Use some common macros for ASCII/EBCDIC

It turns out that the EBCDIC definitions can be made the same as the
ASCII ones, so this moves the ASCII definitions to the spot where other
ones common to the 2 platforms reside, and removes the EBCDIC ones.  In
other words it combines separate definitions into common ones.
handy.h: Comments, white-space only