chasquid: Track and enforce the HELO/EHLO address

HELO and EHLO both take a mandatory parameter, which also should be used
in the Received header.

This patch tracks and enforces that parameter, and also updates the
Received header generation to use it.

https://tools.ietf.org/html/rfc5321#section-4.4

test/t-05-null_address/expected_dsr

@@ -20,7 +20,7 @@ Delivery to the following recipient(s) failed permanently:
 
 ----- Original message -----
 
-Received: from testserver (authenticated as user@testserver)
+Received: from localhost (authenticated as user@testserver)
 	by testserver (chasquid)
 	(over *
 	(envelope from "user@testserver")

test/t-05-null_address/sendmail

@@ -1,4 +1,4 @@
-EHLO
+EHLO localhost
 MAIL FROM: <>
 RCPT TO: user@testserver
 DATA