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When tree.find() results in SUBDOMAIN, the node_path's top contains the
previous node.  Calling previousNode() yet again will return its
further previous node.

This test currently fails with an assertion.

with fixing Conflicts for
	src/lib/dns/gen-rdatacode.py.in
	src/lib/dns/rdata.cc
	src/lib/dns/rrparamregistry-placeholder.cc
	src/lib/dns/tests/rdata_hinfo_unittest.cc
	src/lib/dns/tests/rdata_txt_like_unittest.cc

assert() should be okay, but that depends on details of other classes, so
exception is probably a bit safer.

these are (indirectly) necessary for building external programs.

The lexer variant also uses the string parser eventually (for now).

Also:
* document new_rdata_factory_users[] with an example
* Rename new_rdatafactory_users[] to new_rdata_factory_users[]

not directly related to this task, but I found we need one real (non wrapper)
example to test some of the feature of generic createRdata().

also updated RDATA template including with-lexer constructor.

an empty qstring previously caused an exception, which is a clear bug
and should be fixed.  nul-terminating string regions is an extension,
but I found it useful when implementing RDATA parsers.