router: split router_build_fresh_descriptor() into static functions

Split the body of router_build_fresh_descriptor() into static functions,
by inserting function prologues and epilogues between existing sections.

Write a new body for router_build_fresh_descriptor() that calls the new
static functions.

Initial refactor with no changes to the body of the old
router_build_fresh_descriptor(), except for the split.

Preparation for testing 29017 and 20918.