The file handle is never reset to INVALID_HANDLE_VALUE, and thus the
destructor will assume the operation shall be cancelled and will
delete the temporary file.

This was a major breakage for saving the database file and the state
file.

Necessary for "final" on gcc 4.6.

Several MPD subsystems can now read gzipped files; for example, the
database file can be gzipped.

The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.