Use --dh-param-file option to specify a file including DH parameters
in PEM format.

For example, you can create DH parameters with 1024 bit key using
following command:

$ openssl dhparam -outform PEM -out dhparam.pem 1024

It would be desired to add option to make this limit configurable.
Fix scripted_recv_callback in tests.

The validation rule is described in
http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-02#section-4.1.2

Instead of emitting common headers on decoder side, encoder
now keeps track of removed common headers and encode it
as necessary.