- make assumptions in the spec more accurate
- move some block of comments closer to the corresponding code for readability

- fixed a doxygen line
- added a few corner-case tests

it didn't have to be a loop because of implicit assumptions on how the fields
are to be composed.  the simpler version should be more understandable.
the assumptions are now more explicitly documented.  also, one more test case
was added.

in response to a review comment.

Conflicts:
	src/lib/dns/labelsequence.h

Conflicts:
	src/lib/datasrc/rbtree.h

Also fixes some other API and data differences between master and
trac2092_2 branch which caused several compile and test failures.