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