Simplify the ksba_keyinfo_from_sexp function.

* src/keyinfo.c: Include der-builder.h
(get_ecc_curve_oid): Change to return a string.
(oid_from_buffer): Ditto.
(_ksba_keyinfo_from_sexp): Rewrite.

The DER builder stuff makes things much easier and the code is thus
now actually manageable.

• Signed-off-by: Werner Koch <wk@gnupg.org>