Fix download statistics
Add TODO item: tests
Add make_temporary_directory
Add fd_handle::release()
fd_handle: do not call ::close() on invalid file handle -1
Add file_handle::open(const char *, int, unsigned mode)
Add remove_directory_tree()
Initialize ELF and RPM libraries in test driver
Remove a file with a wrong checksum from the file cache

Also add a few test cases.