Added a mutex to the crypto_algs DH test.
Added a mutex to the crypto_algs DH test.

The goal was to avoid a possible race condition between client and
server.  I thought this was the cause of the test hanging occasionally.
But there appears to be another problem.  I'm guessing the actual
problem has to do with the UNIX domain socket.

joeriggs/experiments,joeriggs/experiments,joeriggs/experiments,joeriggs/experiments