when we don't have a working iconv library we must default to using only builtin...

when we don't have a working iconv library we must default to using only builtin charsets. Defaulting to CP850 when we have just determined that it doesn't work just guarantees that Samba won't work at all (in fact it just seg faults at startup). This fixes Samba on AIX, and I expect on a bunch of other platforms.