Update configuration instructions about config.mk
Lose --usage cli flag
Define VERSION as unknown if it is not supplied

VERSION should be given to the compiler as an argument. If it is not,
define it as "unknown" in the source.
Reorder functions in file.{h,c}
Introduce new IFQ system

Put every input file/dir to input file queue and handle the filetypes
later. The directory queue is now used for postponed directories.
Add get_filetype()