libxl: Enable -Wshadow.

libxl: Enable -Wshadow.

It was convenient to invent $(CFLAGS_LIBXL) to do this.

Various renamings to avoid shadowing standard functions:
  - index(3)
  - listen(2)
  - link(2)
  - abort(3)
  - abs(3)

Reduced the scope of some variables to avoid conflicts.

Change to libxc is due to the nested hypercall buf macros in
set_xen_guest_handle (used in libxl) using the same local private vars.

Build tested only.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>