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/src/thread/pthread_setschedparam.c b/src/thread/pthread_setschedparam.c
new file mode 100644
index 0000000..8e8b5a1
--- /dev/null
+++ b/src/thread/pthread_setschedparam.c
@@ -0,0 +1,10 @@
+#include "pthread_impl.h"
+
+int pthread_setschedparam(pthread_t t, int policy, const struct sched_param *param)
+{
+	int r;
+	__lock(t->killlock);
+	r = t->dead ? ESRCH : -__syscall(SYS_sched_setscheduler, t->tid, policy, &param);
+	__unlock(t->killlock);
+	return r;
+}