convert ns_client and related objects to use netmgr

- ns__client_request() is now called by netmgr with an isc_nmhandle_t
  parameter. The handle can then be permanently associated with an
  ns_client object.
- The task manager is paused so that isc_task events that may be
  triggred during client processing will not fire until after the netmgr is
  finished with it. Before any asynchronous event, the client MUST
  call isc_nmhandle_ref(client->handle), to prevent the client from
  being reset and reused while waiting for an event to process. When
  the asynchronous event is complete, isc_nmhandle_unref(client->handle)
  must be called to ensure the handle can be reused later.
- reference counting of client objects is now handled in the nmhandle
  object.  when the handle references drop to zero, the client's "reset"
  callback is used to free temporary resources and reiniialize it,
  whereupon the handle (and associated client) is placed in the
  "inactive handles" queue.  when the sysstem is shutdown and the
  handles are cleaned up, the client's "put" callback is called to free
  all remaining resources.
- because client allocation is no longer handled in the same way,
  the '-T clienttest' option has now been removed and is no longer
  used by any system tests.
- the unit tests require wrapping the isc_nmhandle_unref() function;
  when LD_WRAP is supported, that is used. otherwise we link a
  libwrap.so interposer library and use that.

bin/tests/system/README

@@ -584,10 +584,6 @@ By default, start.pl starts a "named" server with the following options:
                     preventing multiple instances of this named running in this
                     directory (which could possibly interfere with the test).
 
-In addition, start.pl also sets the following undocumented flag:
-
-    -T clienttest   Makes clients single-shot with their own memory context.
-
 All output is sent to a file called "named.run" in the nameserver directory.
 
 The options used to start named can be altered.  There are three ways of doing
@@ -608,9 +604,9 @@ the named command-line arguments.  The rest of the file is ignored.
 
 3. Tweaking the default command line arguments with "-T" options.  This flag is
 used to alter the behavior of BIND for testing and is not documented in the
-ARM.  The "clienttest" option has already been mentioned, but the presence of
-certain files in the "nsN" directory adds flags to the default command line
-(the content of the files is irrelevant - it is only the presence that counts):
+ARM.  The presence of certain files in the "nsN" directory adds flags to
+the default command line (the content of the files is irrelevant - it
+is only the presence that counts):
 
     named.noaa       Appends "-T noaa" to the command line, which causes
                      "named" to never set the AA bit in an answer.
@@ -635,7 +631,6 @@ certain files in the "nsN" directory adds flags to the default command line
                      the additional section if the response is triggered by RPZ
                      rewriting).
 
-
 Starting Other Nameservers
 ---
 In contrast to "named", nameservers written in Perl or Python (whose script