Allow optional elements in keyinfo objects.

* src/keyinfo.c (_ksba_keyinfo_from_sexp): Allow for optiona elements.
(_ksba_algoinfo_from_sexp): Ditto.

This is useful in case a comment field is part of the the key info.
For example with an ssh key:

(public-key
 (rsa
  (n #00B1[...]#)
  (e #010001#))
 (comment ./test.key))

• GnuPG-bug-id: T4892
• Signed-off-by: Werner Koch <wk@gnupg.org>