news2mail's crosspost handling code generates bad To headers

[eagle]

news2mail uses the following to generate the To header:

print SM "To: @a\n\n";

If there are multiple addresses, this generates a broken header since it uses a bare space as a delimiter. sendmail will then turn this into a dot and corrupt the header. It needs to use a comma instead.

[Julien ÉLIE]

(In [8249]) Add a comma to separate addresses. Otherwise, a broken To:
header was generated by news2mail when multiple addresses
were used.

close #73