The code now avoids any special internal meaning for uid/gid -1, which
allows it to be mapped to a better value (use 4294967295 instead of -1
as the ID to map).  Replaced atol() with something than can return a
value > 0x7FFFFFFF and that will error-out if the value overflows.  If
chown() is called with a uid or gid of -1, complain that the ID is not
settable and signal a transfer error.  Fixes bug 6936.