Upstream: Add snapshot and zvol events

For kernel to send snapshot mount/unmount events to zed.

For kernel to send symlink creates/removes on zvol plumbing.
(/dev/run/dsk/zvol/$pool/$zvol -> /dev/diskX)

If zed misses the ENODEV, all errors after are EINVAL. Treat any error
as kernel module failure.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12416

cmd/zed/zed_exec.c

@@ -26,6 +26,8 @@
 #include <time.h>
 #include <unistd.h>
 #include <pthread.h>
+#include <signal.h>
+
 #include "zed_exec.h"
 #include "zed_log.h"
 #include "zed_strings.h"