Add entity debug field

[necessarily-equal]

This will allow for a better implementation of https://github.com/Unvanquished/Unvanquished/pull/2270

This has an Unvanquished counterpart too, because the intent was to modify entityState_t, and apparently entityState_t has to be kept in sync with playerState_t, and playerState_t lies in Unvanquished. Yep.