emacs: Use whitelist instead of blacklist for term escaping

Previously, the term escaper used a blacklist of characters that
needed escaping.  This blacklist turned out to be somewhat incomplete;
for example, it did not contain non-whitespace ASCII control
characters or Unicode "fancy quotes", both of which do require the
term to be escaped.

Switch to a whitelist of characters that are definitely safe to leave
unquoted.  This fixes the broken test introduced by the previous
patch.