nixos/wireless: use udev to wait for interfaces

I may have finally found a clean solution to the issues[1][2][3] with
the automatic discovery of wireless network interfaces.

[1]: https://github.com/NixOS/nixpkgs/issues/101963
[2]: https://github.com/NixOS/nixpkgs/issues/23196
[3]: https://github.com/NixOS/nixpkgs/pull/125917#issuecomment-856000426

Currently the start script fails right away if no interface is available
by the time it's running, possibly leaving the system without network.
This happens when running a little early in the boot. A solution is to
instead wait for at least one interface to appear before scanning the
/sys/class/net/ directory. This is done here by listening for the right
udev events (from the net/wlan subsystem) using the `udevadm monitor`
command and grep to match its output.

This methods guarantees the availability of at least one interface to
wpa_supplicant, but won't add additional interfaces once it has started.
However, if the current interface is lost, say unplugged, the service is
automatically stopped and will be restarted as soon as a one (not
necessarily the same) is detected. It would be possible make this fully
dynamic by running another service that continously listen for udev
events and manages the main wpa_supplicant daemon, but this is probably
overkill.

I tested the following cases:

  - one interface, starting at boot, w/o predictable naming scheme
  - two interfaces, starting at boot (intel wireless and a usb adapter),
    w/o predictable naming scheme
  - one interface after the system booted, w/o predictable naming scheme
  - two interfaces after the system booted, w/o predictable naming scheme
  - unplugging and plugging back the current interface

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