Overhauled message forwarding, allow comments

No longer derive the new message header from the original - too many problems. Simplified the forwarding support smblib (multiple SENDER* and RECIPIENT* header fields no longer expected/supported). Technically, the FORWARDED header field really serves no important purpose now.
A comment can be passed into forwardmail() or (if NULL), will be prompted for. There is an issue where the added comment can be obscured by the receiving mail program if the original message is a multi-part MIME message. Not sure the best solution for that. Still a WIP in that regard.