sd-varlink: mark functions that can take 'more' flag in IDL structures with an explic...

sd-varlink: mark functions that can take 'more' flag in IDL structures with an explicit flag

Let's mark functions that accept the 'more' flag explicitly for that,
and validate for this explicitly.

This is preparation for
https://github.com/varlink/varlink.github.io/issues/26, if we get that
one day. Let's make sure that from day #1 we have this info available
even if we don't generate this in the IDL for now.

Also enables the two flags for all interfaces we export that use the
logic.