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.h b/parse.h
index 0cdc089..677b62b 100644
--- a/parse.h
+++ b/parse.h
@@ -48,6 +48,7 @@
 	const char *def;		/* default setting */
 	const struct value_pair posval[PARSE_MAX_VP];/* possible values */
 	const char *parent;		/* parent option */
+	int (*verify)(struct fio_option *, void *);
 };
 
 typedef int (str_cb_fn)(void *, char *);