raw-posix: support discard on more filesystems

raw-posix: support discard on more filesystems

Linux 2.6.38 introduced the filesystem independent interface to
deallocate part of a file. As of Linux 3.7, btrfs, ext4, ocfs2,
tmpfs and xfs support it.

Even though the system calls here are in practice issued on Linux,
the code is structured to allow plugging in alternatives for other Unix
variants.  EOPNOTSUPP is used unconditionally in this patch, but it is
supported in both OpenBSD and Mac OS X since forever (see for example
http://lists.debian.org/debian-glibc/2006/02/msg00337.html).

Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>