networking module: init 'wlanInterfaces' option

Configuration option for setting up virtual WLAN interfaces.

If the hardware NIC supports it, then multiple virtual WLAN interfaces can be
configured through the options of the new 'networking.wlanInterfaces' module.
For example, the following configuration transforms the device with the persistent
udev name 'wlp6s0' into a managed and a ad hoc device with the device names
'wlan-managed0' and 'wlan-adhoc0', respectively:

networking.wlanInterfaces = {
    "wlan-managed0" = {
        type = "managed";
        device = "wlp6s0";
    };
    "wlan-adhoc0" = {
        type = "ibss";
        device = "wlp6s0";
    };
};

Internally, a udev rule is created that matches wlp6s0 and runs a script which adds
the missing virtual interfaces and re-configures the wlp6s0 interface accordingly.
Once the new interfaces are created by the Linux kernel, the configuration of the
interfaces is managed by udev and systemd in the usual way.