Inspired by self-recover, sort of. Enabled for unix by default; can
be disabled by commenting out '#define CHECK_PANIC_SAVE' in unixconf.h.
When starting the game, if there is no save file to restore and no
lock/level files to recover, check whether a panic save file exists.
If there is one, tell the player that it's there and that it might be
viable, then ask whether to start a new game.
It doesn't convert the panic save into a reconverable one (rename by
nethack, then continue trying to restore) or tell the player how to
make it viable (rename to remove ".e" by game admin), just whether it
is present. If player opts to start a new game, the panic save is
left alone and will trigger the "there's a panic save file" situation
again once the new game finishes and player starts another.
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