[perl #119123] disallow literal control character variables

This introduces a deprecation warning on things like $^T, where ^T
is a literal control character in the source code.

It includes a minor tweak to how S_scan_ident() skips over whitespace
for non-digit characters, by also skipping over \n -- previously it only
skipped over spaces and tabs.  This shouldn't cause any changes in
behavior, but fixes a bug in how literal control variables are parsed.
That bug can be reproduced on previous versions:

    perl -wle 'eval "\${\cT\n}"; print $@'
perldelta for #119123: disallow literal control character variables