Rename header_offset and header_interval options

Header doesn't really express that this has to do with io verification.
So rename the options to verify_offset and verify_interval.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index e97caf1..eb36d1c 100644
--- a/fio.h
+++ b/fio.h
@@ -395,8 +395,8 @@
 	unsigned int sync_io;
 	unsigned int verify;
 	unsigned int verifysort;
-	unsigned int header_interval;
-	unsigned int header_offset;
+	unsigned int verify_interval;
+	unsigned int verify_offset;
 	unsigned int use_thread;
 	unsigned int unlink;
 	unsigned int do_disk_util;