BUG/MINOR: http-htx: Expect no body for 204/304 internal HTTP responses

BUG/MINOR: http-htx: Expect no body for 204/304 internal HTTP responses

204 and 304 HTTP responses must no contain message body. These status codes are
correctly handled when the responses are received from a server. But there is no
specific processing for internal HTTP reponses (errorfile and http replies).

Now, when errorfiles or an http replies are parsed during the configuration
parsing, an error is triggered if a 204/304 message contains a body. An extra
check is also performed to ensure the body length matches the announce
content-length.

This patch should fix the issue #891. It must be backported as far as 2.0. For
2.1 and 2.0, only the http_str_to_htx() function must be fixed.
http_parse_http_reply() function does not exist.