Cosmetics: clean up header.c
Drop unused header_punt() function

Doesn't look at all useful.
Rename headers_simsendmail to headers_rfc2822

Add comment with the corresponding portion of the table from the RFC.

Make sure all of the limited headers are included and in the same order
for ease of reading, future reference.
Remove debugging code
Clean up logging in header.c. No stderr for you.
Add SUBMISSION_MODE for MSAs, stricter MTAs

In normal MTA mode, behaviour is unchanged. MTA_STRICT checks message
headers for RFC compliance and rejects the message if required headers are
missing or invalid duplicate headers are present. MSA mode will attempt
to add missing required headers that can be automatically generated, in
accordance with RFC 4409.
Implement BOUNCE_LINES
Update CHANGELOG