Add conv=fsync flag to dd

[freqlabs]

Summary

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

Sponsored by: iXsystems, Inc.

[freqlabs]

I am planning a followup to add the fdatasync flag, as well.

[jilles]

Looks good to me.

bin/dd/dd.h

104

This uses up the last free bit, but that should be OK.

[imp]

bin/dd/dd.c

513

do you also need to check close(2) calls?

[freqlabs]

bin/dd/dd.c

513

It's in the stack: https://reviews.freebsd.org/D21376

[freqlabs]

Committed as rS351770