Draft: Don't nullify forwardmove and sidemove when in stasis

A common scripting shortcoming is needing the player's directional input while also needing the player to stay put, but the stasis flags simply do away with it, requiring all sorts of hacky workarounds. The functions P_GetForwardsInput and P_GetSidewaysInput (both source and Lua) were made to represent the input values of cmd.forwardmove and cmd.sidemove when these should be nullified.

This branch is still WIP due to me not yet knowing what to do with the race countdown special case, where it eats every input except the spin button. Suggestions are welcome! :welcometoreleases: