Added smb_getcontenttype(): returns the MIME content-type of the message

or NULL if not relevant (not a MIME-encoded message).
Added smb_countattachments(): returns the number of file attachments in a
MIME-encoded message.
Added SMBMSGTXT_NO_TAILS definition to be used in place of '0' for
common smb_getmsgtxt() mode argument value.
The amount of white-space after "Content-Type:" and the content-type value
is now more flexible (0 or +1 spaces/tabs are fine).