Made new header to be visible by other milters.

- Moved insheader() into EOM (according to http://www.elandsys.com/resources/sendmail/libmilter/smfi_addheader.html)
- Replaced "insheader()" with "addheader()". By unknown reason other milters cannot see headers added by "insheader(,0,,)", but _can_ see headers added by "addheader()".

I've started to use "X-MILTER-DNSBL" header value in other milter and faced, that the header is unreachable. This commit just fixes that.