fsync is a straightforward wrapper around the same C function
(implemented on Windows API as _commit() with identical signature).
From the man pages:
    fsync() transfers ("flushes") all modified in-core data of (i.e.,
    modified buffer cache pages for) the file referred to by the file
    descriptor fd to the disk device (or other permanent storage
    device) so that all changed information can be retrieved even if
    the system crashes or is rebooted.  This includes writing through
    or flushing a disk cache if present.  The call blocks until the
    device reports that the transfer has completed.
RFC: https://wiki.php.net/rfc/fsync_function
Closes phpGH-6650.