libxl: Provide a version of bsd's queue.h as _libxl_list.h

libxl: Provide a version of bsd's queue.h as _libxl_list.h

We would like some linked list macros which are (a) well known to be
sane and (b) typesafe.  BSD's queue.h meets these criteria.

We also provide some simple perlery to arrange to add the libxl_
namespace prefix to the macros.  This will allow us to #include
_libxl_list.h in our public header file without clashing with anyone
else who is also using another version of queue.h.

(A note on copyright: The FreeBSD files we are adding have an
[L]GPL-compatible licence, so there is no need to change our COPYING.
Although FreeBSD's queue.3 still contains the advertising clause, this
has been withdrawn by UCB as recorded in the FreeBSD COPYRIGHT file,
which is included in tools/libxl/external/ for reference.)

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <Ian.campbell@citrix.com>
Tested-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>