zdtm: don't use getpwnam and getgrnam

For us it doesn't matter what names of users and groups are used,
we want to be sure that we restore right uid-s and gid-s.

If we use these functions, we can't run tests in namespaces and
they use an external unix socket, what can be a problem too.

sk unix: 	Runaway socket: ino 0x1df1ae4 peer_ino 0x1df1ae3 family    1 type    1 state  1 name /run/dbus/system_bus_socket
Error (criu/sk-unix.c:712): sk unix: External socket is used. Consider using --ext-unix-sk option.

Cc: Vitaly Ostrosablin <vostrosablin@virtuozzo.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>