MINOR: tools: add a function to load a file into a tar archive

MINOR: tools: add a function to load a file into a tar archive

New function load_file_into_tar() concatenates a file into an in-memory
tar archive and grows its size. Only the base name and a provided prefix
are used to name the faile. If the file cannot be loaded, it's added as
size zero and permissions 0 to show that it failed to load. This will
be used to load post-mortem information so it needs to remain simple.