Add support for postgrey Greylisting

[nurbles]

For folks using postgrey with their postfix mail servers, adding the following lines at line 663 in mailgraph.pl will catch the items greylisted by postgrey:

elsif($prog eq 'postgrey') {
	if($text =~ /\baction=greylist\b/) {
		event($time, 'greylisted');
	}
}

The postgrey home page may be found at https://postgrey.schweikert.ch/ if more information is desired. I have made this mod in my local version twice (so far) after it was replaced by a Ubuntu release upgrade. It would be nice if this little change was built-in.

Hmmm... if I paid more attention I would have been even more puzzled as to why mailgraph does NOT recognize postgrey messages, since both tools were created by the same person!