[bind10-dev] non standard DDNS related checks

[JINMEI Tatuya / 神明達哉]

While updating the description of #1459, I've noticed there are many
additional checks before committing DDNS updates that are implemented
in BIND 9 but not included in the current implementation of BIND 10.

I think #1459 should cover most of such missing checks (I
intentionally omit some of them such as resigning related matters),
but there may be some more.

Not all of them may make sense for BIND 10, but I strongly suggest
someone (which may be me) takes a deeper look at the BIND 9
implementation and learn what it does.  Obviously there should be many
corner cases not directly covered in the RFC, and it's quite unlikely
we notice it if we simply try to re-implement from the scratch.
Unless developing a from-the-scratch implementation itself is a goal
(which I don't think is the case for BIND 10), we should learn from
existing practices.

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JINMEI, Tatuya