fix multiline header parsing
add header parsing tests
enforce the request uri maximum length
enforce the maximum length of various tokens
add rfc 6585 status codes
extract the message body
add a request hook
refactor http connections

A connection works the same way for a server and for a client; the only thing
that will need to be added is a type variable (server or client connection),
and a union to store either a pointer to a struct http_server, or one to a
struct http_client.