scd:nks: Minor additions to the basic IDLM application support.

* scd/app-nks.c (filelist): Use special value -1 for IDLM pubkeys.
(keygripstr_from_pk_file): Handle special value.
(do_readcert): Ditto.
(do_writecert): Ditto.

This allows to get information about the keys from the card. However
the do_readkey still requires a fallback to readcert. This does not
work because there are no certificates yet on the card. The fix is to
fully implement do_readkey.