scd:nks: Emit the algo string with KEYPAIRINFO

* scd/app-nks.c (do_learn_status_core): Emit the algo string as part
of a KEYPAIRINFO.
(struct fid_cache_s): Add field algostr.
(flush_fid_cache): Release it.
(keygripstr_from_pk_file): Fill it and add it to the cache.  Use a
single exit label.  Set algostr.

Backported-from-master: 26da47ae53d51e16ae6867cd419ddbf124a94933
Backported-from-master: 006944b856ee2202905290e8a2f5523a7877d444

This has been backported to keep this, and only this, module in sync
with master. All other changes from the original patch have been
stripped.

• Signed-off-by: Werner Koch <wk@gnupg.org>
• GnuPG-bug-id: T6252, T5144