Improve PKINIT DH output parameter handling

Improve PKINIT DH output parameter handling

Apply current practices for output parameter handling and memory
management to client_create_dh(), client_process_dh(), and
server_process_dh().  Initialize the output arguments at the
beginning, use local variables to hold their values until success is
guaranteed, and transfer memory to the output arguments at the end.
Use a cleanup label which runs on both success and failure.

The client_create_dh() cleanup code conditionalizes on retval, which
we usually try to avoid, as it needs to clean up a cryptoctx field on
error only.

[ghudson@mit.edu: wrote commit message; added similar changes to
client_create_dh() and client_process_dh()]