ptrace: ifdef PTRACE_LISTEN

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
plugins: std -- Drop main() calculation

We don't need to calculate main() address manually, the linker
should have done it for us.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>