Add appropriate fsync calls.

[kr]

Should have optional throttling. There should be three modes: always fsync (for best reliability), fsync at most once every N seconds, or never fsync (best performance).

Could be controlled by two options: "-s 0" for always fsync, "-s N" for sync at most once every N seconds, and "-S" for never fsync.

[kr]

Add fsync options.

Closed by f54b3d9.