Fix up check for high-bit-set characters, which provoked "comparison is
always true due to limited range of data type" warnings on some compilers,
and was unlike the way we do it elsewhere anyway.  Fix omission of "$"
from the set of valid identifier continuation characters.  Get rid of
sanitize_text(), which was utterly inconsistent with any other error report
anywhere in the system, and wasn't even well designed on its own terms
(double-quoting the result string without escaping contained double quotes
doesn't seem very well thought out).  Fix up error messages, which didn't
follow the message style guidelines very well, and were overly specific in
situations where the actual mistake might not be what they said.  Improve
documentation.

(I started out just intending to fix the compiler warning, but the more
I looked at the patch the less I liked it.)