<sched.h> should offer both __sched_priority and sched_priority.

<sched.h> should offer both __sched_priority and sched_priority.

If glibc hadn't already done things this way round, I'd have
called the field sched_priority and the macro __sched_priority
since that would seem less likely to cause trouble, but glibc
source compatibility is probably more important.

Change-Id: I8a8a477f2aa87cae641069c5c84b4fcab3152a82