Merge branch 'damage-control' into 'master'

Damage control + Match rebalancing

This is two branches in one since while I was working on damage-control's changes months back, I felt it was best Match rebalancing was merged in here too (thanks JTE for helping me do so).

Changes from damage-control:
* `player->health` (formerly the "HUD copy" of `player->mo->health`) is now `player->rings`, and is also now the player's actual ring count.
* `player->mo->health` (formerly rings + 1) is now always 1 when alive, regardless of ring count; if player with rings is damaged, this is untouched.
* P_RingDamage now includes ring spilling code.
* P_ShieldDamage now has a damagetype argument, allowing me to remove the last MT_NULL hack left in from the pre-damagetype days that I forgot about.
* The old "switch-to-seestate" enemy damaging behavior in P_DamageMobj has been removed. This was a Doom left-over and doesn't really affect SRB2's enemies anyway - see, Doom enemies had a random chance of using seestate or painstate, SRB2 enemies always use painstate.
* Other minor efforts to reorganise damaging code and have it make more sense, but otherwise nothing that should affect gameplay in general.

Changes from match-rebalancing:
* New weapon/ammo dropping behavior: if you have the weapon panel + ammo, you drop the panel (but not the ammo); if you don't, you just drop the ammo.
* New Match ammo consumption: Weapon rings can now be fired with no rings at double the ammo cost.
* New emerald behaviour: collecting all 7 emeralds no longer turns you super (read: Match super is dead now) but instead steal points from enemies and gives you and teammates invincibility + sneakers
* Tails ringslinger buff: Any character with CA_FLY will now throw rings 1.5x as fast.

See merge request !28