unix: add Linux watchdog structures and missing constants

This CL also adds the type-safe wrappers IoctlGetWatchdogInfo
and IoctlWatchdogKeepalive around the raw ioctl interface.

Change-Id: I98e3970807b28832e49524700b17b6b6f75d91ed
Reviewed-on: https://go-review.googlesource.com/c/sys/+/259497
Run-TryBot: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Terin Stock <terinjokes@gmail.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Tobias Klauser <tobias.klauser@gmail.com>

unix/linux/types.go

@@ -120,6 +120,7 @@ struct termios2 {
 #include <linux/stat.h>
 #include <linux/taskstats.h>
 #include <linux/tipc.h>
+#include <linux/watchdog.h>
 #include <linux/vm_sockets.h>
 
 // abi/abi.h generated by mkall.go.
@@ -2510,3 +2511,7 @@ const (
 	CAN_RAW_FD_FRAMES     = C.CAN_RAW_FD_FRAMES
 	CAN_RAW_JOIN_FILTERS  = C.CAN_RAW_JOIN_FILTERS
 )
+
+// Watchdog API
+
+type WatchdogInfo C.struct_watchdog_info