Add O_DIRECT flag to DD for cache bypass

[rscheff]

Summary

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.

Test Plan

Mount an NFS export. Perform dd reads twice in a row and validate
that traffic is observed on the network in the 2nd iteration also.