libthread_xu: Properly inherit the creating thread's sched_priority.

libthread_xu: Properly inherit the creating thread's sched_priority.

When a new thread is created with the inheritsched attribute set to
PTHREAD_INHERIT_SCHED, then it must inherit the sched_priority
scheduling parameter from the creating thread, because POSIX sez:

"The following thread scheduling attributes defined by POSIX.1-2008
are affected by the inheritsched attribute: [...], scheduling parameters
(schedparam), [...]"

Also:

"For SCHED_FIFO and SCHED_RR, the only required member of the sched_param
structure is the priority sched_priority. For SCHED_OTHER, the affected
scheduling parameters are implementation-defined."

For SCHED_OTHER, it will currently inherit the creating thread's
sched_priority too. We'll see if that poses a problem.