scd:nks: Some code cleanup.
* scd/app-nks.c (find_fid_by_keyref): Factor keyref parsing out to ... (parse_keyref): new. (do_readcert): Use new function instead of partly duplicated code. Make detection of keygrip more robust. (do_readkey): Make detection of keygrip more robust. (do_with_keygrip): Use get_nks_tag.
Also added a couple of comments.
(cherry picked from commit b92b3206e72b635fd815eaf85e7acc67c2a52ffe)