Support log2_cofactor for Edwards curves (NFC).

This works more or less like the similar refactoring for Montgomery
curves in 7fa0749fcb: where we previously hardwired the clearing of 3
low bits of a private exponent, we now turn that 3 into a curve-
specific constant, so that Ed448 will be able to set it to a different
value.

ssh.h

@@ -476,6 +476,7 @@ struct ec_ecurve
     EdwardsCurve *ec;
     EdwardsPoint *G;
     mp_int *G_order;
+    unsigned log2_cofactor;
 };
 
 typedef enum EllipticCurveType {