libzfs{,_core}: set O_CLOEXEC on persistent (ZFS_DEV and MNTTAB) fds

These were fd 3, 4, and 5 by the time zfs change-key hit
execute_key_fob()

glibc appends "e" to setmntent() mode, but musl's just returns fopen()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11866

lib/libzfs_core/libzfs_core.c

@@ -137,7 +137,7 @@ libzfs_core_init(void)
 {
 	(void) pthread_mutex_lock(&g_lock);
 	if (g_refcount == 0) {
-		g_fd = open(ZFS_DEV, O_RDWR);
+		g_fd = open(ZFS_DEV, O_RDWR|O_CLOEXEC);
 		if (g_fd < 0) {
 			(void) pthread_mutex_unlock(&g_lock);
 			return (errno);