Refactor NamedInstance.rndc() to use EnvCmd() interface

Refactor NamedInstance.rndc() to use EnvCmd() interface

To unify the command handling, utilize EnvCmd() to handle rndc commands:

1. Remove isctest.rndc abstractions. They were intended for an upcoming
   python-only implementation. A couple of years later, it doesn't seem
   to be coming any time soon, so let's stick with the interface that
   makes sense today, i.e. use the same command handling interface
   everywhere.
2. Remove the specialized rndc.log in favor of the generic logging
   already implemented by isctest.run.cmd(). I believe the cause of the
   many rndc(log=False) invocations was that nobody wanted this extra
   file. Yet, logging everything by default makes sense for debugging,
   unless there's a good reason not to. In almost all cases, logging was
   switched to the default (enabled).
3. With the NamedInstance.rndc() call now returning CmdResult rather
   than combined stdout+stderr string, adjust all the invocations to use
   `.out` or `.err` as necessary.
4. Replace some manual rndc invocation and its base argument
   construction with the standardized nsX.rndc() call.
5. In cases where rndc is expected to fail, utilize
   raise_on_exception=False and check the `.rc` from the result, rather
   than handling an exception.
6. In addzone/tests_rndc_deadlock.py, refactor the test slightly to
   avoid using EnvCmd() entirely to avoid spamming the logs. This test
   calls rndc in a loop from multiple threads and such test case is an
   exception which doesn't warrant changing the `isctest.run.cmd()`
   implementation.