Sort Well Fed foods by item name

The Well Fed item list is unsorted, and may change sorting unpredictable
as you select different foods. Fix this by sorting the foods by their
item name.

This adds a utility function Cork.SortedPairs() which can be used to
iterate over any table by its keys in sorted fashion.
Provide a stable ordering for corks

Sort the corks by their name instead of by the internal ordering of the
Lua table.

Also fix battleground functionality. The recent changes to cork sorting
started reading a global named `inbg` instead of actually testing for
the battleground. We were also filtering out nobg corks twice.