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.
Hmmm. That's the wonderful thing about external state :cry:. Does the browser check the global nssdb before the user one? Now that you have the script it's peanuts to run it at system activation to maintain the global nssdb.

Another solution is to force-use `/run/current-system/sw` for the library so that the current version is a global state setting.