Rework the valid_number_field test function to be more

Rework the valid_number_field test function to be more
lenient.  There are an enormous number of tar writing
programs, many which don't quite follow the specs.

Note:  I don't think I recall seeing any that left
number fields with trailing garbage; I may be wrong
about that, in which case, this would have to stop
at the first NUL (after the first octal character).