CBufferedFile: convert into a non-refcounted RAII wrapper

- it now takes over the passed file descriptor and closes it in the
  destructor
- this fixes a leak in LoadExternalBlockFile(), where an exception could
  cause the file to not getting closed

- disallow copies (like recently added for CAutoFile)
- make nType and nVersion private
CBufferedFile: add explicit close function

- also use identical close function for CAutoFile (avoids setting file to
  NULL under wrong conditions)
CAutoFile: make file private
Merge pull request #4978

938bcce CAutoFile: make file private (Philip Kaufmann)
0c35486 CBufferedFile: add explicit close function (Philip Kaufmann)
c9fb27d CBufferedFile: convert into a non-refcounted RAII wrapper (Philip Kaufmann)