(to_oct): New parameter substitute, giving a substitute value to use

when the original value is out of range.  Do not append a space to the
output; modern tars don't.  When a value is out of range, specify the
maximum value, not the number of bits.
(GID_NOBODY, UID_NOBODY): New macros.
(gid_to_oct, uid_to_oct): Use them as substitutes.
(finish_header): Do not assume that UINTMAX_TO_OCT appends a space.
(dump_file): Check whether the file changed as we read it.