General NiGHTS Fixes

I held off on this until I'd finish another merge request first, but that won't be finished anytime soon, so there's little reason to delay this further.

This merge request cherry-picks the following changes from !1792 (closed):
- Flying diagonally down-right now uses a flight angle of 315 instead of 314 (This is a hack, to compensate for player->sidemove and player->forwardmove being +35,-36 when pressing down-right, which yields an angle of 314)
- Increment DEMOVERSION, and check for older demo versions to use the old code when watching replays
- When in reverse gravity, the player sprite angle is no longer mismatched with the flight angle
- Optimize angledif2 out of P_NiGHTSMovement() (technically might have a difference if Lua sets player->flyangle above 359?), and un-loop a for loop by incrementing by thrustfactor directly
- DBG_NIGHTSBASIC-devmode now displays "Angle: [player->flyangle]" above "Drill: [player->drillmeter]"
- DBG_NIGHTSBASIC-devmode no longer displays a very faint 4294967295 (-1) link value when you don't have a link

TL;DR: The only change that'd be relevant for changelog readers is "In NiGHTS mode, fixed the player being angled incorrectly in reverse gravity", which is also almost not worth mentioning.