sched/: Fix several inappropriate accesses to get_errno() that were missed in...

sched/:  Fix several inappropriate accesses to get_errno() that were missed in previous changes (some going back to nuttx-.23).  Add new nxsched_setscheduler() and nxsched_getaffinity() which are equivalent to their counterparts without the nx on front.  These versions do not modify the errno value.  Changed all calls within the OS to use these newer versions of the functions.