Add the NULL ammo type to prevent debug messages

http://smf.cataclysmdda.com/index.php?topic=5155

This also disables the display of the ammo type if the type is "NULL", but maybe it should also disable the display of the other ammo properties (damage, recoil, dispersion, armor-pierce, range) as they don't apply to seeds, or do they?

Changes the ammo-type of towels, still and folding_bicycle to "NULL" instead of "none". An ammo type of "none" makes it possible to "Unload" towels: The item menu shows unloading of towels as possible (green U), but attempting to do so results in a debug message "Tried to get invalid ammunition: none". Same applies to reloading it.