Add "Ammo per shot" DEHACKED field

[XaserAcheron]

Adds a new Ammo per shot field to DEHACKED. Implementation mirrors Eternity's, for forward-compat n' whatnot.

A summary of sorts:

• Weapons now use this field to determine if the player has enough ammo to fire, instead of hardcoding the value for each weapon.
• Weapon autoswitch has been fixed in general to check the correct ammotype & ammo use for each weapon, rather than assume the vanilla ammotypes (this is an ooooold bug from ever since DEHACKED was introduced).
• The new A_ConsumeAmmo and A_CheckAmmo MBF21 DEHACKED codepointers will use the ready weapon's Ammo per shot field if the amount arg is set to zero.
• Vanilla doom weapon fire codepointer behavior varies on whether or not the Ammo per shot field is explicitly set on a weapon.
  • If set, the ready weapon's Ammo per shot field is subtracted from the weapon's ammo type.
  • If not set, the amount of ammo subtracted depends on the weapon codepointer being used (e.g. A_FireShotgun2 will subtract 2, A_FireBFG will subtract BFGCELLS -- this mirrors vanilla Doom behavior)
• For general sanity, the "BFG Cells/Shot" field will also set the BFG weapon's Ammo per shot field.

There's a little bit of refactoring of things that touches a bit of Heretic (namely, all the various ammo-per-shot arrays for the doom/heretic weapons were replaced with the new weaponinfo_t ammopershot field ), though this feature is not yet fully implemented for Heretic 'cause it doesn't do DEH. ;) -- I tested Heretic to make sure it all works as intended still (and the spec passes), so all should be well there.

[Will update mbf21.md here in a bit; need to make the PR first. ;]

[XaserAcheron]

Quick-ish update: needed to add a check for wp_noammo in P_SubtractAmmo, and while I was in the neighborhood, I went ahead and made it so in mbf21 ammo cannot go into the negatives, 'cause nobody wants that. :P

[XaserAcheron]

Just updated mbf21.md with some info on the new stuffs.

[kraflab]

I'll resolve the conflicts, sec