Don't repeate mail "comment" headers in exported netmail msgs

smb_getmsgtxt() returns any "comment" headers as part of the message body text, and has since 2003.
When I added mail->netmail (stored message) export support to SBBSecho v3, I duplicated this logic, so comment headers were added *twice* to exported netmail messages. Oops. I didn't notice until today when playing with forwarding-to-netmail (FTN).

Also noticed and fixed: any comment headers in posted messages (unlikely/uncommon) would not be exported to EchoMail messages. Use the correct smb_getmsgtxt() mode flags ("NO_TAILS" instead of "BODY_ONLY") - I know, not great names.