Changed the way yaz-client encodes charset negotiation information

Changed the way yaz-client encodes charset negotiation information
to target when no negoatitionCharset is set by user using command
negcharset. In YAZ 3.0.0 and later the negotation information was
simply omitted. Now the OriginProposal is sent anway but with
proposedCharSets and proposedlanaguages omitted (both are optional).
YAZ Frontend server is also modified to deal with this: omitted
proposedCharSets in the OriginProposal will make the frontend server
return default character set for target if that is supplied by the
backend init handler (member query_charset).