Doomsday Engine

Beginning investigation into the source of the dangling/unused file
identifiers left behind prior to an en engine reset:

• Instrument file indentifier addition/release in _DEBUG builds with
  verbose >= 1
• Explicitly clear identifiers in F_Shutdown to ensure we don't leak.

It would appear that one of the danglers is doomsday.ded while the
others appear to be due to bookkeeping error (duplicated invalid id).