site.c: Allocate and free the Diffie--Hellman secret buffers each time.

site.c: Allocate and free the Diffie--Hellman secret buffers each time.

This will allow us to maybe use a different Diffie--Hellman group, with
different size requirements, to communicate with the same peer at a
later time.  We can't do this yet, of course, but the negotiation
machinery is coming soon.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>