"prompt" and subsequent first "label" were not separated
in any way while second "label" and forth were; let's make
the resulting isolinux.cfg a tiny bit more pretty.

This one was suggested by enp@ for industrial use where
some extra protection for the boot process might be quite
desirable.

If no syslinux ui was specified (the stock configuration paths
ensure there is one) or if it was set to "none" explicitly,
then there's no boot: prompt (let alone any menu).

If there's a need to ensure that the boot process is not
interruptable by Ctrl/Shift/Caps Lock/Scroll Lock.