scripts: don't rely on "stat -" support

scripts: don't rely on "stat -" support

While commit b72682c602b8 ("scripts: Use stat to check lock claim")
validly indicates that stat has gained support for the special "-"
command line option in 2009, we should still try to avoid breaking being
able to run on even older distros. As it has been determined, contary to
the comment in the script using /dev/stdin (/proc/self/fd/$_lockfd) is
fine here, as Linux specially treats these /proc inodes.

Suggested-by: Ian Jackson <ian.jackson@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
Release-acked-by: Paul Durrant <paul@xen.org>