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