Add conv=fdatasync flag to dd

The fdatasync flag performs an fdatasync(2) on the output file before closing it.
This will be useful for the ZFS test suite.

Submitted by: Ryan Moeller
Reviewed by: manpages, mmacy@
MFC after: 1 week
Sponsored by: iXSystems, Inc.
Differential Revision: https://reviews.freebsd.org/D21373