Adds back the ability to avoid using DH params, provided no file was explicitly supplied.
This used to be `DHPARAM_GENERATION=false`, the equivalent is now `DHPARAM_SKIP=1` (default 0). Previous name was no longer appropriate.
Ensures that if a user has explicitly provided their own dhparam file to still output a warning instead of the skip message, since `DHPARAM_SKIP=1` doesn't disable the support in nginx.