nsenter: add tests

Signed-off-by: Andrey Vagin <avagin@openvz.org>
nsenter: check errors from open(), read() and open()

Currently if nsenter is executed without /proc, it segfaulted.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
nsenter: return an error if a process with specified pid doesn't exist

Signed-off-by: Andrey Vagin <avagin@openvz.org>
nsenter: return an error if a process with specified pid is a zombie

Signed-off-by: Andrey Vagin <avagin@openvz.org>