nextboot: warn about limitations of /boot/nextboot.conf on ZFS

MFC after:	1 week

sbin/reboot/nextboot.sh

@@ -50,6 +50,14 @@ if [ ${force} = "NO" -a ! -d /boot/${kernel} ]; then
 	exit 1
 fi
 
+df -Tn "/boot/" 2>/dev/null | while read _fs _type _other ; do
+	[ "zfs" = "${_type}" ] || continue
+	cat 1>&2 <<-EOF
+		WARNING: loader(8) has only R/O support for ZFS
+		nextboot.conf will NOT be reset in case of kernel boot failure
+	EOF
+done
+
 cat > ${nextboot_file} << EOF
 nextboot_enable="YES"
 kernel="${kernel}"