extract: add some facility for extensible header parsing We need a way for extension to extend the header we can parse. We start with a very simple mechanism that will be used in the next changeset.

extract: remove the 'user' variable It is not heavily used enough to justify being something other than a dictionary entry.

extract: use a single return The differences between both returns are now very thin, we factor out that part.

extract: move 'nodeid' assignment where it is parsed The is one setter and no consumer, we can move it there directly.