add support for thread scheduling (POSIX TPS option)

linux's sched_* syscalls actually implement the TPS (thread
scheduling) functionality, not the PS (process scheduling)
functionality which the sched_* functions are supposed to have.
omitting support for the PS option (and having the sched_* interfaces
fail with ENOSYS rather than omitting them, since some broken software
assumes they exist) seems to be the only conforming way to do this on
linux.
diff --git a/include/pthread.h b/include/pthread.h
index 74d8600..660a64d 100644
--- a/include/pthread.h
+++ b/include/pthread.h
@@ -91,6 +91,10 @@
 void pthread_testcancel(void);
 int pthread_cancel(pthread_t);
 
+int pthread_getschedparam(pthread_t, int *__restrict, struct sched_param *__restrict);
+int pthread_setschedparam(pthread_t, int, const struct sched_param *);
+int pthread_setschedprio(pthread_t, int);
+
 int pthread_once(pthread_once_t *, void (*)(void));
 
 int pthread_mutex_init(pthread_mutex_t *__restrict, const pthread_mutexattr_t *__restrict);