Add oflag=fsync and oflag=sync capability to dd

Sets the O_FSYNC flag on the output file. oflag=fsync and oflag=sync are
synonyms just as O_FSYNC and O_SYNC are synonyms. This functionality is
intended to improve portability of dd commands in the ZFS test suite.

Submitted by: Ryan Moeller
Reviewed by: manpages, mmacy@
MFC after: 1 week
Sponsored by: iXsytems, Inc.
Differential Revision: https://reviews.freebsd.org/D21422