Add eid-mw: authenticate & sign with Belgian eID identity cards

Adds limited but useful support for Belgian electronic ID cards.

Limited for now, because:
- While a Firefox extension is included and installed, no effort is made to integrate it. I use Chromium.
- The library should work with all NSS-compatible browsers, but must be registered manually by and for each user. Instructions are included.
(Sigh. It turns out that Chromium completely ignores /etc/pki and only uses ~/.pki. I assume that a patch to Chromium and Wrapper integration will be required to automagically integrate all this on NixOS.

I'd like to keep that separate issue for a subsequent patch, though. Per-user configuration does work without issues, and hopefully other browsers are less broken.)

> Another solution is to force-use /run/current-system/sw for the library

That's what the script currently does, except it silently falls back to the user profile if eid-mw isn't globally installed.

I could add a warning to the effect that the setting is hard-linked to either the system or user profile, if that's what you mean...