[Dovecot] deliver copies "^From " header lines when delivering to Maildir

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.