regcomp.c: Add capability for stricter parsing of []

regcomp.c: Add capability for stricter parsing of []

This adds the capability, currently unused, of forbidding certain things
in [bracketed] character classes.  Included are things that warn bug
still compile, such as false ranges, [\d-\w], and unrecognized escapes.

Also forbidden are potentially ambiguous cases where \x (without braces)
isn't followed by exactly two hex digits, or \000 where the number of
octal digits isn't precisely three.