Add O_DIRECT flag to DD for cache bypass

FreeBSD DD utility has not had support for the O_DIRECT flag, which
is useful to bypass local caching, e.g. for unconditionally issuing
NFS IO requests during testing.

Reviewed by: rgrimes (mentor)
Approved by: rgrimes (mentor, blanket)
MFC after: 3 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D25066