Ignore missing Q in dh_params

Some implementations don't send the required Q value in dh_params, so
allow it to be absent.
Simplify pkinit_check_dh_params

Change pkinit_check_dh_params() to take two DH* parameters, and only
compare p and g, because q is fully determined by them and might be
missing.

Also refactor some parameter checks into check_dh_wellknown() that
were previously done separately in the pkinit_process_td_dh_params()
and server_check_dh().