deliver appears to copy "^From " header lines from stdin when delivering
to Maildir.  The header filtering in mbox-save.c prevents this when
delivering to an mbox and instead a synthesized "^From " header is
written.

This is an issue when using, for example, Postfix and

  mailbox_command = /usr/libexec/dovecot/deliver

which does not appear to have an option not to write a "^From " header on
the mailbox_command input.

I'm attaching a patch that adds a save_header_callback() along the same
lines as in mbox-save.c but I'm really not sure this is the right way to
do it.
