Split/Fold the To/CC header fields to avoid reaching the 998 characters limit

[jmrouet]

If svnnotify is used to send emails to too many users it can create a non compliant To: header line.
This can also happen easily if multiple -x arguments are used. Then some email adresses can be repeted several times in the To: header field.

I suggest to

1. split (aka fold) the To: header line with CRLF as described in RFC822.
2. remove duplicate email adresses in both $to or $cc fields.

Thanks for the excellent job
Regards,
Jean-Michel.