common: Extend the new get_keyalgo_string function

* common/openpgp-oid.c (openpgp_oid_or_name_to_curve): New.
(get_keyalgo_string): Use it.

We do not always have an OID, so except the name or the alias of the
curve as well. This creates a second entry mapping to the same name
but that does not matter.

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