Add dhparam sanity check and update DH_check documentation

The -check argument to dhparam should never identify any problems if we
have just generated the parameters. Add a sanity check for this and print
an error and fail if necessary.

Also updates the documentation for the -check argument, and the DH_check()
function.

RT#4244

Reviewed-by: Richard Levitte <levitte@openssl.org>