smb_getmsgtxt() can now return *just* the plain-text part of a multipart MIME
encoded message by setting the GETMSGTXT_PLAIN bit in the mode argument.
This is useful for reading (many) MIME encoded emails via the Terminal server.
The full MIME body text is left in place for those message viewing methods
which can nicely handle (e.g. display) non-plain-text parts.