Expansion of item_flags

Both items and itemtypes now have item_flags, so flags can now be set for specific items.

One example of this use would be to heat up food, which may (or may not) provide a morale boost (fixes #200).

Other uses which can be implemented:
- rotten (instead of the current method which determines this state from item's bday).  Potentially will solve cooked food becoming unrotten problem.
- blunt (for cutting weapons)
- fitted clothing (to minimise itypedef clutter)