unix: add Klogset and related SYSLOG_ACTION_ constants

Adds a new function: Klogset which wraps sys_syslog for
cases where the `bufp` argument is not required and the
argument that follows needs to be passed to the function.

Fixes golang/go#34672

Change-Id: Idf026791ea9265aaca33e78a34fcd490edb8d326
Reviewed-on: https://go-review.googlesource.com/c/sys/+/199278
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -1851,6 +1851,17 @@ func OpenByHandleAt(mountFD int, handle FileHandle, flags int) (fd int, err erro
 	return openByHandleAt(mountFD, handle.fileHandle, flags)
 }
 
+// Klogset wraps the sys_syslog system call; it sets console_loglevel to
+// the value specified by arg and passes a dummy pointer to bufp.
+func Klogset(typ int, arg int) (err error) {
+	var p unsafe.Pointer
+	_, _, errno := Syscall(SYS_SYSLOG, uintptr(typ), uintptr(p), uintptr(arg))
+	if errno != 0 {
+		return errnoErr(errno)
+	}
+	return nil
+}
+
 /*
  * Unimplemented
  */