include client_priv.h in mysqltest.c.  Portability fix.


client/client_priv.h:
  Include client_priv.h, for WEXITSTATUS definition.
  
  Remove code which is already in client_priv.h.
  
  Rearrange OPT_* enum to co-exist with options defined in client_priv.h.
client/mysqltest.c:
  Move WEXITSTATUS #define from mysqltest.c to client_priv.h, as it is
  useful in other clients (in particular, current mysql_upgrade.c won't
  compile without it on some platforms).
  
  Add a final OPT_MAX_CLIENT_OPTION to enum options_client, which can
  be used when a maximum shared client option value is needed.