fix some problems in control-spec

  with an internal ERROR message.

  The client may decline to provide a body for the original address, and

  instead send a special null address (0.0.0.0 for IPv4, ::0 for IPv6, or "."

  for hostname).  This signifies that the server should choose the original

  address itself, and return that address in the ADDRESSMAPPED message.  The

  server should an element of address space that is unlikely to be in actual

  use.  If there is already an address mapped to the destination address, the

  server may reuse that mapping.

  instead send a special null address (0.0.0.0 for IPv4, ::0 for IPv6, or

  "."  for hostname).  This signifies that the server should choose the

  original address itself, and return that address in the ADDRESSMAPPED

  message.  The server should ensure that an element of address space

  that is unlikely to be in actual use.  If there is already an address

  mapped to the destination address, the server may reuse that mapping.

  If the original address is already mapped to a different address, the old

  mapping is removed.  If the original address and the destination address