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/sched/sched_setscheduler.c

@@ -1,8 +1,8 @@
 #include <sched.h>
+#include <errno.h>
 #include "syscall.h"
 
 int sched_setscheduler(pid_t pid, int sched, const struct sched_param *param)
 {
-	static const struct sched_param def;
-	return syscall(SYS_sched_setscheduler, pid, 0, &def);
+	return __syscall_ret(-ENOSYS);
 }