smb_getplaintext and smb_getmsgtxt(...,GETMSGTXT_PLAIN) will now fall-back

to a text/html part of a multi-part MIME encoded message if no text/plain part
exists.
Changed smbmsg_t.charset to smbmsg_t.text_charset.
Added smbmsg_t.text_subtype (e.g. "plain" or "html").
Fixed issue with parsing parsed plain-text when the charset was the last
element of the MIME-part header.