helper: use unix socket for local api

bitmask-vpn installs a daemon called bitmask-helper
which exposes a http api to handle firewall up/down
and openvpn connect/disconnect (only in macos)

this daemon was listening on localhost, since it is
meant to be only accessible to local processes we'v
changed it to listen on a unix domain socket

using unix socket allows to apply file  permissions
and restrict access to the api to only the intended
user