* core program logic

    what's the file type?

        call lstat (l is needed to handle symbolic links)
        file_stat->st_mode tells you the file type

    how to handle the entries

        1. write header
            populate all of the header fields that are for reg files
            write the header as a 512 block
        2. if it's a regular file; write the file contents
            open file
            while:
                read a 512 byte block from the source
                    if read returns 0, already at EOF -- done
                write the 512 bte block to the archive
                    if read returns < the requested size, now at EOF -- done
        3. if it's a directory
            open dir
            while more entries:
                handle the entry

    what happens with very deeply nested directory trees?

       open and opendir calls could fail if you run out of FDs, but on
       a modern system that's unlikely to happen -- you'll run out of
       space in prefix first

    At the end, write two empty blocks (or more, GNU tar rounds up to an
    particular number of blocks -- see the manual or man page)

* some weirdnesses

    GNU tar does UID and GID differently when it's a large number (see
    the latest version of the homework spec for details)

    See "The default blocksize ..." in the standards page description of the
    ustar format. The full size of a compliant tar file after writing the
    two empty logical records to terminate the file MUST be an even multiple
    of the default blocksize.
    
    The magic and version values from GNU tar are different fromt what's
    specified in the pax standard for the ustar format.