exposed ply.startboost and the absent BT_ flags to lua

ply.startboost (or player->startboost in this context) is completely missing from the player lua definitions, aswell as some BT_ button flags that could be useful for lua addons (both are absolutely necessary for the addon im working on atm).

this merge should add both ply.startboost and the remaining BT_ button flags to lua. ill admit i havent tested this at all, but until i figure out how to compile anything i probably never will.

my bad if this doesnt work lol - king the memer |^|