MINOR: log: provide a function to emit a log for a session

The new function sess_log() only needs a session to emit a log. It will
ignore the parts that depend on the stream. It is usable to emit a log
to report early errors in muxes. These ones will typically mention
"<BADREQ>" for the request and 0 for the HTTP status code.
diff --git a/include/proto/log.h b/include/proto/log.h
index 419e0b3..7fb1f04 100644
--- a/include/proto/log.h
+++ b/include/proto/log.h
@@ -78,6 +78,7 @@
  * Will not log if the frontend has no log defined.
  */
 void strm_log(struct stream *s);
+void sess_log(struct session *sess);
 
 /*
  * add to the logformat linked list