nixos/wpa_supplicant: escape interface names to listen on

Systemd provides some functionality to escape strings that are supposed
to be part of a unit name[1]. This seems to be used for interface names
in `sys-subsystem-net-devices-{interface}.device` and breaks
wpa_supplicant if the wireless interface name has a dash which is
encoded to \x2d.

Such an interface name is rather rare, but used i.e. when configuring
multiple wireless interfaces with `networking.wlanInterfaces`[2] to have on
interface for `wpa_supplicant` and another one for `hostapd`.

[1] https://www.freedesktop.org/software/systemd/man/systemd-escape.html
[2] https://nixos.org/nixos/options.html#networking.wlaninterfaces