Install client Diffie-Hellman parameters into initramfs.

* debian/control (Package: mandos-client/Recommends): Added
  "gnutls-bin | openssl" (to generate DH parameters).
* debian/mandos-client.postinst (create_dh_params): New function
                                                    called on package
                                                    configure.
* plugins.d/mandos-client.c (main): Bug fix: Work around Debian bug
                                    #633582 also for the
                                    "client-dhparams.pem" file.