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- include style ("" vs <>) per consensus
- null pointer comparison style
- documentation wording fix

as a zone writer can only update a single zone.

as it's not only used for reloading (or not even for "loading").

This also makes ZoneWriter a concrete class (and removes
ZoneWriterLocal).

Clarify high-level design of IPC

it's a leftover of an intermediate attempt of testing the case; on a closer
look, testing send_command turned out to be better and sufficient.

It was mostly an example and is now merged to the base version. No
longer needed.

also confirmed it would fail without the change of this branch.  the execution
of bindctl would fail, so bind10_control.py needed to be updated so it can
ignore failure of bindctl if specified so.