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.
Since the zone now has a reference to their respective configuration
tree, `rndc showzone` can be used for any zones (including those
defined in namedconf), without `allow-new-zones` being enabled.
Add a test for this.
The test is part of the addzone suite because showzone used to be
related to addzone, but this could be moved elsewhere in the future
if more specific tests are needed for showzone.
