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.
Actually, in shell this should be written as `if [ -n "$2" ]; then` because if `$2` is `-h` or whatever it will be parsed and give mysterious error messages. Not really a big deal in this case though.