fix signed-compare warnings

The m_iSelection array could probably be made unsigned... IF we were
confident we never try to assign it a negative number, of which fact I'm
not convinced.
remove unnecessary enum-to-int cast
fix logic/crash in CoinModeNoHome option

This option was returning -1 for an index and triggering an assert, and
the logic in the other direction was typoed.
ensure that GetCreditsMessage returns a value

The unhandled case: in Free Play mode and players can't join.  Can this
even happen?
remove accidentally shadowing declaration

With this in place, the orgSel was always Invalid.
fix type-punning warning

This should be a no-op (though I wouldn't be offended if someone checks
me on that).  I think the code's clearer without casting back and forth
anyway.
Merge branch 'warnings' of https://github.com/djpohly/stepmania into djpohly-warnings