Add the ability to pass a NULL output buffer and/or an unlimited

output length to charset_{to,from}_unicode, permitting convenient
dry-running of conversions to determine the required output length
and/or test for the presence of difficult characters.

git-svn-id: svn://svn.tartarus.org/sgt/charset@7677 cda61777-01e9-0310-a592-d414129be87e