Simplify the handling of optparamlen and RFC9072 support.

Simplify the handling of optparamlen and RFC9072 support.

Use the ibuf_size() of the optional params (opb) when writing the various
headers. Only use optparamlen to write the Opt Parm Len field of the OPEN
message. Since there the size could be forced to 255 in case of a RFC9072
exended header. Also use size_t for optparamlen since the new ibuf API
no longer depends on using the right sized type.
OK tb@