textonly: emit the produced body length for Content-Length recomputation

Adding "-w <file>" emits the computed resulting content-length once the
message has been entirely sent. The body bytes really produced are
counted and the total written to the designated file. The message body
output now goes through put_body_str()/put_body_ch() which adapt to the
requested behavior: without -w nothing is written, so textonly stays a
plain filter usable on large mailboxes.