Support forwarding of single-part HTML emails, add "Fwd:" subject prefix

When forwarding a single-part MIME-encoded HTML email, the preamble (original message header info) and any user comments, need to be HTML-encoded.

Add the commonly-used "Fwd: " prefix to the default message subject, when forwarding.

This required that smb_getplaintext() no longer always-NULLify the message's text_subtype (e.g. "html"). For single-part messages, this element was getting freed and NULLed.

Add/use a new SMBLIB convenience function to add a string header field, but only if non-NULL: smb_hfield_string()