ROUTERS:

The yield of a router is one of:

  OK              the address was routed and either added to one of the
                  addr_local or addr_remote chains, or one or more new
                  addresses were added to addr_new. The original may be added
                  to addr_succeed.

  REROUTED        this is used when a child address is created and added to
                  addr_new as a consequence of a domain widening or because
                  "self = reroute" was encountered. The only time it is handled
                  differently from OK is when verifying, to force it to
                  continue with the child address.

  DECLINE         the address was not routed; pass to next router unless
                  no_more is set. It is permitted for additional addresses to
                  have been added to addr_new (or indeed for addresses to have
                  been put on the other chains).

  PASS            the address was not routed, but might have been modified;
                  pass to the next router unconditionally.

  DISCARD         the address was discarded (:blackhole: or "seen finish")

  FAIL            the address was not routed; do not pass to any subseqent
                  routers, i.e. cause routing to fail.

  DEFER           retry this address later.

Both ERROR and DEFER cause the message to be kept on the queue; either may
request freezing, but nowadays we try not to request freezing from routers
because it may hold up other addresses in the message.


When routing succeeds, the following field in the address can be set:

  transport       points to the transport instance control block

  uid, gid        are the uid and gid under which a local transport is to be
                  run if the transport does not itself specify them.

  initgroups      is set true if initgroups() is to be called when using the
                  uid and gid set up by the router.

  fallback_hosts  fallback host list - relevant only if the router sets up
                  a remote transport for the address.

  errors_address  where to send error messages for this address.

  extra_headers   additional headers to be added to the message for this
                  address.

  remove_headers  the names of headers to be removed from the message for this
                  address

****
