In preparation for switching linuxulator to the use the native 1:1
threads split sys_sched_getparam(), sys_sched_setparam(),
sys_sched_getscheduler(), sys_sched_setscheduler() to their kern_*
counterparts and add targettd parameter to allow specify the target
thread directly by callee.

Differential Revision: https://reviews.freebsd.org/D1034
Reviewed by: trasz