Tolerate curve25519 backends where the high bit of the pk isn't ignored
Right now, all our curve25519 backends ignore the high bit of the public key. But possibly, others could treat the high bit of the public key as encoding out-of-bounds values, or as something to be preserved. This could be used to distinguish clients with different backends, at the cost of killing a circuit. As a workaround, let's just clear the high bit of each public key indiscriminately before we use it. Fix for bug 8121, reported by rransom. Bugfix on 0.2.4.8-alpha.