Add configure flags for gfapi and rbd

Add --disable-{gfapi,rbd} to disable gfapi/rbd support even if available.

Signed-off-by: Jens Axboe <axboe@fb.com>
Verify: Tighten header length check in verify_header()

Fix-up to commit 5964842c: Tighten the header length check as well, don't only
change the message.