Better check of DH parameters in TLS data

When the client reads DH parameters from the TLS stream, we only
checked that they all are non-zero.  This change updates the check to
use DH_check_params()

DH_check_params() is a new function for light weight checking of the p
and g parameters:

    check that p is odd
    check that 1 < g < p - 1

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>