Add Lua bindings for MUSICDEF (read-only, of course)

Satisfies Issue #1157

local mInfo = S_MusicInfo() -- info about the currently playing music

print(mInfo.name)
print(mInfo.title)
print(mInfo.alttitle)
print(mInfo.authors)
print(mInfo.soundtestpage)
print(mInfo.soundtestcond)
print(mInfo.stoppingtics)
print(mInfo.bpm)
print(mInfo.loop_ms)
print(mInfo.allowed)