Merge branch 'jt/trailer-with-cruft'

Update "interpret-trailers" machinery and teaches it that people in real world write all sorts of crufts in the "trailer" that was originally designed to have the neat-o "Mail-Header: like thing" and nothing else. * jt/trailer-with-cruft: trailer: support values folded to multiple lines trailer: forbid leading whitespace in trailers trailer: allow non-trailers in trailer block trailer: clarify failure modes in parse_trailer trailer: make args have their own struct trailer: streamline trailer item create and add trailer: use list.h for doubly-linked list trailer: improve const correctness