Add parent link to options

Then we can nest display for --cmdhelp=all, to tie options
together.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/parse.h b/parse.h
index 752da34..c3b66d7 100644
--- a/parse.h
+++ b/parse.h
@@ -45,6 +45,7 @@
 	const char *help;		/* help text for option */
 	const char *def;		/* default setting */
 	const struct value_pair posval[PARSE_MAX_VP];/* possible values */
+	const char *parent;		/* parent option */
 };
 
 typedef int (str_cb_fn)(void *, char *);