On OS/X (and possibly other systems implementing F_FULLFSYNC), also ask the
hardware to perform a sync (like on Windows), instead of only syncing the
data to the hardware buffers.

This matches how the sync works in sqlite and may cause a performance hit
when other data is waiting to be written in the hardware buffers, as most
disks only allow syncing the entire write buffer at once.

* subversion/libsvn_subr/io.c
  (includes): Try to include sys/ioctl.h

  (svn_io_file_flush_to_disk): Use full sync when possible.

See also:
http://lists.apple.com/archives/darwin-dev/2005/Feb/msg00072.html

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1484439 13f79535-47bb-0310-9956-ffa450edef68