Expose some globals to Lua

Barrels O'Funrequested to merge
BarrelsOFun/SRB2:expose_nummaprings into next

This MR exposes these global variables

nummaprings : read & write; should allow scripters better control over a map's ring count.

cursaveslot : read only, returns -1 in netgames; allows per save slot I/O for Lua scripts.

gamedatafilename : read only, returns gamedata without the file extension.

Edited by Barrels O'Fun

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