threadobj_set_priority() is replaced and superseded by
threadobj_set_schedparam(), for changing the scheduling parameters of
a thread. Unlike its predecessor, this new service does not decide of
the target scheduling class (depending on the priority level), which
is left to the caller instead.

As a notable side-effect, threadobj_set_rr() is dropped, since
enabling round-robin basically means switching to SCHED_RR via a call
to threadobj_set_schedparam(), and disabling it happens implicitely
whenever a thread leaves the SCHED_RR class.