Properties in the "user" setting are a convenient way to associate any
kind of user-provided metadata to connections.
However, nmcli doesn't support the user setting at the moment and
adding this feature requires a significant effort. Without nmcli
support, dispatcher scripts can only access user properties by either
parsing connection files or by using D-Bus (with or without libnm and
GObject introspection). Since both these solutions are not very
convenient, provide an alternative way: pass the properties as
environment variables.