* solver7 server not exiting, despite saying ok

* the dictionaries or some signature of them needs to be part of
  the cache name, so that solver7 can know *itself* if the cache
  needs deleting and regenerating (and so that if the '--dict-*'
  options are used then the wrong cache won't be used.
* test with word length 2 and 3
* rename the solver7-4 and solver7-5 caches accordingly

* re-implement solver6 as a wrapper around solver7 but calling
  specific dictionaries
* test with word length 2 and 3

* Messages consist of a 'command' item and an 'argument_list' item.
  Rename these to 'command' and 'supplementary_data' respectively.
* Replies are scalar. Change this (in server() and send_message_...())
  to be 'status' and 'supplementary_data'.
* If 'status' is 'OK' then proceed as currently, but in the
  cases where there server can't process the message (e.g. 'cos
  supplementary_data missing) then it should reply 'ERROR' and
  set supplementary_data to be the error message string. The client
  can then detect this and call miniade.error(supplementary_data).
* test with word length 2 and 3