shared/verbs: allow multiple verbs to be handled by a single function

shared/verbs: allow multiple verbs to be handled by a single function

With the uintptr_t data parameter, it is actually quite nice to have
VERB(do_impl, "name-a", …)
VERB(do_impl, "name-b", …)
int do_impl(…) { … }

To make this work, the do_impl_data struct needs to have a unique name and
we also need to suppress the warning about the forward declaration for
do_impl being repeated. I think it's fine to suppress the warning, it's
not needed for anything. If somebody declares the function with the same
name by mistake, the implementations are going to conflict too.