Autoload charnames for \N{name}

Autoload charnames for \N{name}

This autoloads charnames.pm when needed.  It uses the :full and :short
options.  :loose is not used because of its relative unfamiliarity in
the Perl community, and is slower.  (If someone later added a typical
"use charnames qw(:full)", things that previously matched under :loose
would start to fail, causing confustion.  If :loose does become more
common, we can change this in the future to use it; the converse isn't
true.)

The callable functions in the module are not automatically loaded.  To
access them, an explicity "use charnames" must be provided.

Thanks to Tony Cook for doing a code inspection and finding a missing
SPAGAIN.