Item description and metabolic interchange bugfix

Fixes #8584 - check for enough free power capacity before adding power. It will still add "chunks" of 25 power every 5 turns, but it wont get you above the maximal power capacity. If the character has 24 power units less than the the maximum, it will not gain any power, but given that the minimal capacity is 100 (normal power storage CBM), the character will go up to at least 76 power units.

Fixes #8539 - dispersion for guns was compared wrong, same for some properties of ammo.
Lower dispersion is better, but a higher armor-pierce, damage and range value is better (was the inverse before).
Also combine the code for ammo and ammo in containers, and changed the description for `it_ammo::count`. Currently is says "Count", but this can be mistaken for the stack count of the item itself. It actually is only the default stack size (e.g. when being spawned), or crafted.