25519 is 255-bit?

[NIIBE Yutaka]

I don't know if we should "fix" this or not, but I think that
it's 255-bit.

GnuPG uses this information when it tries to send private key to
smartcard.  In development version of Gnuk, the key attribute says
it's 255-bit.  And I encounter the mismatch.

Here is a patch to show the issue.  It is no problem for me to modify
Gnuk side.

diff --git a/cipher/ecc-curves.c b/cipher/ecc-curves.c
index 0f622f7..13e043e 100644
--- a/cipher/ecc-curves.c
+++ b/cipher/ecc-curves.c
@@ -118,7 +118,7 @@ static const ecc_domain_parms_t domain_parms[] =
   {
     {
       /* (-x^2 + y^2 = 1 + dx^2y^2) */
-      "Ed25519", 256, 0,
+      "Ed25519", 255, 0,
       MPI_EC_EDWARDS, ECC_DIALECT_ED25519,
       "0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFED",
       "-0x01",
--