Direct syscalls using syscall.Syscall(SYS_*, ...) should no longer be
used on darwin, see [1]. Instead, use the fcntl libSystem wrappers
provided by the golang.org/x/sys/unix package which implement the same
functionality.
[1] https://golang.org/doc/go1.12#darwin
A call file.Sync on OSX doesn't guarantee actual persistence on
physical drive media as the data can be cached in physical drive's
buffers. Hence calls to file.Sync need to be replaced with
fcntl(F_FULLFSYNC).