MEDIUM: http: add a pointer to the buffer in http_msg

MEDIUM: http: add a pointer to the buffer in http_msg

ACLs and patterns only rely on a struct http_msg and don't know the pointer
to the actual data. struct http_msg will soon only hold relative references
so that's not possible. We need http_msg to hold a reference to the struct
buffer before having relative pointers everywhere.

It is likely that doing so will also result in opportunities to simplify
a number of functions arguments. The following functions are already
candidate :

	http_buffer_heavy_realign
	http_capture_bad_message
	http_change_connection_header
	http_forward_trailers
	http_header_add_tail
	http_header_add_tail2
	http_msg_analyzer
	http_parse_chunk_size
	http_parse_connection_header
	http_remove_header2
	http_send_name_header
	http_skip_chunk_crlf
	http_upgrade_v09_to_v10