Revert "Make curve25519-donna-c64 handle non-canonical points like the ref implementation."

This reverts commit bd8cbd3175cd2041945ea28bf0d4521fa79de26b.

Conflicts:
	src/libsodium/crypto_scalarmult/curve25519/donna_c64/smult_curve25519_donna_c64.c
	test/default/scalarmult7.c
	test/default/scalarmult8.c
Revert bd8cbd3175cd2041945ea28bf0d4521fa79de26b
Go back to masking the high bit in curve25519 instead of processing the
key as a 256-bit value.
Replace curve25519 ref implementation with latest ref10 from Supercop.