Make the %s format (seconds since the epoch) work for a negative

Make the %s format (seconds since the epoch) work for a negative
number and when used with a zero-padded field width, e.g. %015s.

(my_strftime): Move the `do_number_sign_and_padding'
label so that it precedes the code to set `digits'.  Otherwise,
%0Ns wouldn't work.  Before this change, `date -d @-22 +%05s' would
print `00-22'.  Now, it prints `-0022', as it should.