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.

src/thread/pthread_getschedparam.c

@@ -0,0 +1,17 @@
+#include "pthread_impl.h"
+
+int pthread_getschedparam(pthread_t t, int *restrict policy, struct sched_param *restrict param)
+{
+	int r;
+	__lock(t->killlock);
+	if (t->dead) {
+		r = ESRCH;
+	} else {
+		r = -__syscall(SYS_sched_getparam, t->tid, &param);
+		if (!r) {
+			*policy = __syscall(SYS_sched_getscheduler, t->tid);
+		}
+	}
+	__unlock(t->killlock);
+	return r;
+}