common: Rename an odd named function.

* common/openpgp-oid.c (oid_crv25519): Rename to oid_cv25519.
(openpgp_oid_is_crv25519): Rename to openpgp_oid_is_cv25519.  Change
callers.

We use "cv25519" everywhere else and thus the test function should not
have a surprising name.

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