Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Don't assign same value.

Eliminate duplicated code.

Remove 3rd clause, renumber, ok per email

use rtprio_thread system call to get or set thread priority.

Same as pthread_setschedparam, use sizeof(struct sched_param) instead.

Caching scheduling policy and priority in userland, a critical but baddly
written application is frequently changing thread priority for SCHED_OTHER
policy.

Use thr_setscheduler, thr_getscheduler and thr_setschedparam to implement
pthread functions.

Use kernel facilities to support real-time scheduling.

WARNS level 4 cleanup.

- Add libthr but don't hook it up to the regular build yet. This is an
adaptation of libc_r for the thr system call interface. This is beta
quality code.