Added a standard mopher header

Mopher can now be configured to add a standard header, containing a few useful
symbols. Configuration:

  # Add the mopher standard header
  mopher_header = 0

  # Header name
  mopher_header_name = "X-mopher"

CAVEAT: Because smfi_chgheader can only be called from xxfi_eom, this only
works if a messages passes the eom stage.