regcomp.c: Debug output clearer ranges

regcomp.c: Debug output clearer ranges

It's not immediately obvious what the character class [!-~] matches.
Better is its equivalent: [\x21-\x7e].  This commit changes the debug
output to be the latter for character class matches, while retaining the
current behavior where it is clear what the range matches, in, e.g.,
[J-R].  Ranges like [A-z] include more than just alphabetics, so they
are now output as [\x41-\x7a].  (Debug output is done, for example, when
the command line option -Dr is specified.)