add flag to ruleset that indicates a message is forwarded, slightly modifying how junk analysis is done

part of PR #50 by bobobo1618

testdata/smtp/junk/domains.conf

@@ -11,6 +11,13 @@ Accounts:
 						SMTPMailFromRegexp: remote@example\.org
 						AcceptRejectsToMailbox: mjl2junk
 						Mailbox: mjl2
+			mjl3@mox.example:
+				Rulesets:
+					-
+						SMTPMailFromRegexp: remote@forward\.example
+						VerifiedDomain: forward.example
+						IsForward: true
+						Mailbox: mjl3
 		RejectsMailbox: Rejects
 		JunkFilter:
 			# Spamminess score between 0 and 1 above which emails are rejected as spam. E.g.