Add verify hook to options

This allows us to individually verify options, instead of having
a big function that attempts to fix things up.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/parse.c b/parse.c
index 1b38db5..5043374 100644
--- a/parse.c
+++ b/parse.c
@@ -428,6 +428,12 @@
 		ret = 1;
 	}
 
+	if (ret)
+		return ret;
+
+	if (o->verify)
+		ret = o->verify(o, data);
+
 	return ret;
 }