common: New helper functions for OpenPGP curve OIDs.

* common/openpgp-oid.c (openpgp_oidbuf_to_str): Factor most code out
to ...
(openpgp_oidbuf_to_str): new.
(openpgp_oidbuf_is_ed25519): New.
(openpgp_oidbuf_is_cv25519): New.

At some places it is more convenient (and faster) to directly work on
buffers and avoid the way via opaque MPIs. These 3 new functions
allow for that.

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