Doomsday Engine

• AbstractFile now holds an instance of LumpInfo. All file types
  now share this common base component which stores file/lump
  metadata such as absolute path, size, last-modification time etc...
• LumpFile no longer needs to hold an instance of LumpInfo as this
  is now provided by the abstract base.
• Moved DFILE file open flag up into AbstractFile. Convention is
  that if AbstractFile cannot return a handle then the resource
  is not presently "open".
• Renamed GL_DestroyImagePixels as GL_DestroyImage (standardize)
• Fixed duplicate Loading <resourcename>... log messages output
  when verbose >= 1 due to reentrancy into F_Open when testing
  for attempts to load duplicate files.

Todo: F_Open being recursively reentrant is not exactly ideal,
despite the elegance of the solution. A fair amount of work gets
done twice for every file-open with the no-duplicate predicate.