Switch from gen_dh to link_dh

The link_dh is new command that allows linking pre-generated dhparam file instead of generating new one. There is still an option to generate new one and of course previous one is not touched. This applies only for new certificates. There should be no issue in using pre-generated dhparam and at the same time is is huge time saver.

Fixes #23 (closed)

Requires turris/turris-os-packages!707