Add socket setting

[SamyDjemai]

When the tailscaled socket is at a non-default path, for example /tmp/tailscaled.sock when ran in a container in userspace mode, tailscale-manager fails to update the routes.

The syntax for the command becomes:

tailscale --socket=/tmp/tailscaled.sock set --advertise-routes=...

We should be able to provide tailscale-manager with a socket setting in its config file or as an argument, in order to point to the desired socket.