unix: add SchedSetAttr and SchedGetAttr for Linux

Add wrappers for sched_getattr(2) and sched_setattr(2), as well as
various SCHED_ values usable for these.

The kludge in linux/types.go is needed so we can include both
linux/sched/types.h (for struct sched_attr) and sched.h (for a few
defines from include/bits/cpu-set.h).

Unfortunately, they both define struct sched_param, thus the need to
mask one of the definitions.

Change-Id: I3e13cf49ccef7ae81a75d33826d18de84a52106d
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/sys/+/516756
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>

unix/linux/types.go

@@ -138,6 +138,11 @@ struct termios2 {
 #include <linux/random.h>
 #include <linux/rtc.h>
 #include <linux/rtnetlink.h>
+// This is to avoid a conflict of struct sched_param being defined by
+// both the kernel and the glibc (sched.h) headers.
+#define sched_param kernel_sched_param
+#include <linux/sched/types.h>
+#undef kernel_sched_param
 #include <linux/shm.h>
 #include <linux/socket.h>
 #include <linux/stat.h>
@@ -5796,3 +5801,7 @@ const (
 	RISCV_HWPROBE_MISALIGNED_UNSUPPORTED = C.RISCV_HWPROBE_MISALIGNED_UNSUPPORTED
 	RISCV_HWPROBE_MISALIGNED_MASK        = C.RISCV_HWPROBE_MISALIGNED_MASK
 )
+
+type SchedAttr C.struct_sched_attr
+
+const SizeofSchedAttr = C.sizeof_struct_sched_attr