Rework COPYING installed in /usr/include/xen/, due to several licences

Rework COPYING installed in /usr/include/xen/, due to several licences

The notice in the COPYING file in "xen/include/public/COPYING" doesn't
really apply to the files that ultimately are been install at
"/usr/include/xen". The issue are headers in the "sys/" subdirectory
that comes from other projects such as Linux or FreeBSD.

The main issue is that there are two headers that have a different
licence than the MIT licence:

- xen-sys/Linux/gntalloc.h (installed as "sys/gntalloc.h") is public
  domain.
- xen-sys/FreeBSD/gntdev.h (installed as "sys/gntdev.h") is BSD-2.

To clarify this, we'll install a COPYING file with a different notice.

Reported-by: Andrew Cooper <Andrew.Cooper3@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>