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/agents/zfs_retire.c

@@ -40,6 +40,7 @@
 #include <sys/fm/fs/zfs.h>
 #include <libzfs.h>
 #include <string.h>
+#include <libgen.h>
 
 #include "zfs_agents.h"
 #include "fmd_api.h"