Consistent separation of responsibilities between nord and openvpn

[jbweston]

At the moment we pass basically no flags to openvpn, which means that we have to handle many things in Python code (e.g. if we want some action to be performed when openvpn starts up or goes down, or we want to reconnect when the connection goes down).

However, openvpn itself has many flags that can control this behaviour (--up, --down, ...).
The advantage of using these flags is that it is less likely that bugs in nord will cause the system to be left in an inconsistent state (e.g. DNS settings not reset).

[jbweston]

As a part of this we should have a more clear separation of privileges also.

At the moment we need to be able to execute tee, cat, openvpn and kill under sudo.
If we tell OpenVPN to drop privileges (with the --user and --group flags) then we do not need sudo to kill the process. The disadvantage is that, as the openvpn process is a child of the sudo process that launched it we need to do some gymnastics to get the PID of the actual openvpn process.