Add killswitch flag, automatic reconnection

[Darkren]

Did you run make format && make check?

Fixes #575

Changes:

• --killswitch flag was added. If set, Internet won't be restored during VPN session re-establishment. Default value is false;
• VPN client now tries to automatically re-establish session in case on connection failure. Keeps trying until app is stopped;
• Command to fetch default network interface. Previous one sometimes returned wrong value.

How to test this PR:
Run VPN adding killswitch flag to client app like this: "--killswitch=true"