Check connection state by name

[MeneDev]

Currently the VPN must be the only tun device and it must be a tun device (other modes are not supported).

This is a limitation that exists only in the check for an already established connection. The check could also be performed by querying nmcli. This was originally avoided as it creates a dependency on nmcli. However, the dependency also exists for establishing the connection.

nmcli should also be used to query the connection state as this allows querying for the name rather than the device type only.