Allow multi-digit numbers in enhanced SMTP status codes.

The re_DSN regular expression is intended to match a response like
"550 1.2.3", where "1.2.3" is an enhanced status code. However, the
regular expression was only expecting single-digit numbers between the
dots; thus it was missing codes like "550 5.7.23". The expression has
been updated to accept any number of digits between the dots.

This fixes unmatched rejection lines that begin with,

  550 5.7.23 <user@example.com>: Recipient address rejected...

Such lines appear with newer versions of the python SPF policy daemon.