[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/proxy.h b/include/types/proxy.h
index 1c73d24..88a01cc 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -258,6 +258,11 @@
 	int no_options2;			/* PR_O2_* */
 
 	struct pxcounters counters;		/* statistics counters */
+
+	struct {
+		const char *file;		/* file where the section appears */
+		int line;			/* line where the section appears */
+	} conf;					/* config information */
 };
 
 struct switching_rule {