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 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 a new address was added
              to addr_new, or the address was discarded because it
              generated an address that appears higher up the parent
              chain. Note that a local address is permitted to specify
              a remote transport.
              
  FAIL        the address was not directed; pass to next director.
              
  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

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. 

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