add new gbsplay features to xgbsplay

[mmitch]

Based on #72, #74 and others, new features have appeared in gbsplay.
They should be added to xgbsplay as well.
This list might be out of date, it is more of a general reminder:

• add single-song loop mode (shared code, -L works automatically)
• update xgbsplay help text (shared code, works automatically)
• add interactive loop-mode selection
• display loop mode
• display pause
• update xgbsplay manpage