Forbid variable names with ASCII non-graphic chars

Forbid variable names with ASCII non-graphic chars

See http://nntp.perl.org/group/perl.perl5.porters/229168

Also, the documentation has been updated beyond this change to clarify
related matters, based on some experimentation.

Previously, spaces couldn't be in variable names; now ASCII control
characters can't be either.  The remaining permissible ASCII characters
in a variable name now must be all graphic ones.