sched: mark several kern.sched.* sysctls as CTLFLAG_RWTUN

The following sysctls which are not touched during boot time
initialization are marked as CTLFLAG_RWTUN so they can be set by loader
tunables as well:

• kern.sched.interact
• kern.sched.preempt_thresh
• kern.sched.static_boost
• kern.sched.idlespins
• kern.sched.balance
• kern.sched.steal_idle
• kern.sched.steal_thresh
• kern.sched.trysteal_limit
• kern.sched.always_steal

MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50279