[MINOR] config: reference file and line with any listener/proxy/server declaration

Those will be used later for cross-references of conflicts or errors.
diff --git a/include/types/server.h b/include/types/server.h
index ddf5a46..45528fa 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -131,6 +131,11 @@
 	int puid;				/* proxy-unique server ID, used for SNMP */
 
 	struct srvcounters counters;		/* statistics counters */
+
+	struct {
+		const char *file;		/* file where the section appears */
+		int line;			/* line where the section appears */
+	} conf;					/* config information */
 };