also be more strict of 'target': we don't (yet) support non NULL case for it
in this Python version anyway, so it must be None.  Added a check for it.
docstring was also updated accordingly.

rename addWildcardNxrrsetProof to addWildcardNXRRSETProof
(for consistency with addNXDOMAINProof())

Conflicts:
	ChangeLog

also introduce a separate log message ID for XfrinProtocolError at the
INFO level (ERROR isn't appropriate because these are generally uncontrollable
events), and made sure we use the XfrinProtocolError for errors due to
the remote behavior.

code by extracting the validation part into a separate method.

(there are some corner cases that haven't been addressed yet)

as it will be used for other purposes than IXFR.  If it's not found,
just log it and set it to None instead of raising an exception; what
should happen depends on how it's used, so it now lets the later process
decide what to do in that case.

and tests.

reviewed on jabber