MAJOR: http: make http_msg->sol relative to buffer's origin

msg->sol is now a relative pointer just like all other ones. There is no
more absolute references to the buffer outside the struct buffer itself.

Next two cleanups should include removing buffer references to functions
which already have an msg, and removal of wrapping detection in request
and response parsing which cannot wrap by definition.
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 2c8b876..d680658 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -311,7 +311,7 @@
 	unsigned int next;                     /* pointer to next byte to parse, relative to buf->p */
 	unsigned int sov;                      /* current header: start of value */
 	unsigned int eoh;                      /* End Of Headers, relative to buffer */
-	char *sol;                             /* start of line, also start of message when fully parsed */
+	unsigned int sol;                      /* start of line, also start of message when fully parsed */
 	unsigned int eol;                      /* end of line */
 	unsigned int som;                      /* Start Of Message, relative to buffer's origin */
 	int err_pos;                           /* err handling: -2=block, -1=pass, 0+=detected */