ipdb: check external calls in compat module

Now the compat module checks the exit status of external
processes (like brctl).

Bug-Url: https://github.com/svinota/pyroute2/issues/45
Bug-Url: https://github.com/svinota/pyroute2/issues/46
Bug-Url: https://github.com/svinota/pyroute2/issues/47