If we can't init audio, disable it

In order to detect that, initialize just the pygame modules we need.
Fix average shields stat calculation

Whoops, that was a silly bug.
Change order of kill statistics

Make it match the order in which the races are defined in the theme,
like it used to before save files started saving in alphabetical order.
More sentry fixes

Don't try to load nonexistant sentry data, don't force a "sentry" option
in instant action, and actually delete the "kills-sentry" stat from a
save file if the theme doesn't have them.
Add "strict character stats" option

Instead of always deleting kill statistics for races not in the current
theme, do so depending on whether or not the "strict character stats"
option is enabled.
Keep track of if the player saved recently

When is either saved or loaded, the "just_saved" variable is set to
True; upon doing anything that would result in a change to the character
file, it gets set to False. The appearance of the main_menu_confirm
screen is now dependent on this variable.