nixos/pcscd: Improve and clean up module

So far the module only allowed for the ccid driver, but there are a lot
of other PCSC driver modules out there, so let's add an option called
"plugins", which boils down to a store path that links together all the
paths specified.

We don't need to create stuff in /var/lib/pcsc anymore, because we
patched pcsclite to allow setting PCSCLITE_HP_DROPDIR.

Another new option is readerConfig, which is especially useful for
non-USB readers that aren't autodetected.

The systemd service now is no longer Type=forking, because we're now
passing the -f (foreground) option to pcscd.

Tested against a YubiKey 4, SCR335 and a REINER SCT USB reader.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @wkennington