Implement read-only mode for SSIDs from the additional config (-I)

Implement read-only mode for SSIDs from the additional config (-I)

On NixOS[1] - a Linux distribution which allows to configure a full OS
declaratively - it's possible to configure SSIDs for `wpa_supplicant`
like this:

    networking.wireless.networks = {
      myssid = {
        pskRaw = "<redacted>";
      };
    };

It's also possible to add networks "imperatively" using `wpa_gui` or
`wpa_cli`. However it's not possible to do both because if the first
option is used, NixOS creates a read-only symlink at
`/etc/wpa_supplicant.conf` and then it's not possible for
`wpa_supplicant` anymore to write to it.

This patch aims to help us changing this: while "declarative" SSID
configuration can be quite useful, it's a bad idea for e.g. sensitive
stuff like a WPA2 enterprise network.

The original idea was to use `-I`[2] for immutable configs (including
"declarative" networks) on NixOS and `-c /etc/wpa_supplicant.conf` for
anything "imperative".

However this doesn't really work out because if a wifi network from a
config file specified with `-I` is changed by e.g. `wpa_gui`, it's
silently overwritten in `/etc/wpa_supplicant.conf` (specified with
`-c`) which is IMHO unintuitive (in our case at least). This patch
basically declares each network defined in a config file passed via `-I`
to `wpa_supplicant` as "read-only" and doesn't write these "read-only"
networks to `/etc/wpa_supplicant.conf`.

A bit more context can be found on GitHub in the PR where I implemented
this[3].

[1] https://nixos.org/
[2] Added in e6304cad47251e88d073553042f1ea7805a858d1
[3] https://github.com/NixOS/nixpkgs/pull/113716

Signed-off-by: Maximilian Bosch <maximilian@mbosch.me>