DIRECTORS:

On entry, the "local_part" field of the address points to the local part
we are dealing with, with any prefix and/or suffix removed.

The "domain" field points to the local domain; this has been checked for
applicability to this director. The "require_files" option has not been
checked; this is to allow for the inclusion of $home in the file names for
those directors for which it is relevant.

The yield of a director is one of:

  OK          the address was directed and either added to one of the
              addr_local or addr_remote chains, or one or more new addresses
              were added to addr_new. Note that a local address is permitted
              to specify a remote transport.

  FAIL        the address was not directed; pass to next director. 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).

  FORCEFAIL   the address was not directed; do not pass to any subseqent
              directors.

  DEFER       retry this address later.

  ERROR       there was some bad error while directing this address (usually a
              problem with the director's configuration).

Both ERROR and DEFER cause the message to be kept on the queue; either may
request freezing.

When direction suceeds, the following fields in the address may be set:

  transport   points to the transport instance control block; this can
              be a local or a remote transport.

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

  home_dir    the "home" & current directory for transporting, if the transport
              does not specify one itself.

****
