(relativize): New function.

(relativize): New function.
> (relativize): New function, with much of old start_header's guts.
Handle filesystem prefixes.
(start_header): Use this new function.
(init_sparsearray): Don't bother to zero out the new array; it's not needed.
(deal_with_sparse): Fix array allocation bug.
(create_archive): Don't assume '/' is the only separator.
(dump_file): Likewise.
Don't worry about leading / in symlink targets.