Mask curve25519 keys during generation (before serializing them)

This was broken in #922 (merged as part of #956).

This would cause GPG to be unable to parse unencrypted secret keys,
thinking they were encrypted.

rfc4880bis-08 hints at this requirement, saying:

o  MPI of an integer representing the secret key, which is a scalar
   of the public EC point.

Since scalar multiplication happens after masking the private key,
this implies that we should serialize the private key after masking,
as well.

src/crypto/public_key/elliptic/curves.js

@@ -191,6 +191,8 @@ Curve.prototype.genKeyPair = async function () {
       return nodeGenKeyPair(this.name);
     case 'curve25519': {
       const privateKey = await random.getRandomBytes(32);
+      privateKey[0] = (privateKey[0] & 127) | 64;
+      privateKey[31] &= 248;
       const secretKey = privateKey.slice().reverse();
       keyPair = nacl.box.keyPair.fromSecretKey(secretKey);
       const publicKey = util.concatUint8Array([new Uint8Array([0x40]), keyPair.publicKey]);