message_stream: deduce default envelope address from the current user email

* libmailutils/stream/message_stream.c: Unless specified otherwise,
set envelope address based on the current user email.
* mh/tests/burst.at: Remove all X-Envelope headers before comparison.

@@ -236,13 +236,15 @@ _message_open (mu_stream_t stream)
 	  mu_address_t addr;
 	  
 	  mu_address_create (&addr, from);
-	  if (!addr
-	      || mu_address_aget_email (addr, 1, &env_from))
-	    env_from = mu_strdup ("GNU-Mailutils");
+	  if (addr)
+	    {
+	      mu_address_aget_email (addr, 1, &env_from);
 	      mu_address_destroy (&addr);
 	    }
-      else
-	env_from = mu_strdup ("GNU-MH");
+	}
+
+      if (!env_from)
+	env_from = mu_get_user_email (NULL);
     }
   free (from);
   

@@ -40,16 +40,15 @@ Sergey
 ])
 
 burst +inbox 1 || exit $?
-grep -v ^X-Envelope-Date Mail/inbox/2
-grep -v ^X-Envelope-Date Mail/inbox/3
-grep -v ^X-Envelope-Date Mail/inbox/4
+grep -v ^X-Envelope- Mail/inbox/2
+grep -v ^X-Envelope- Mail/inbox/3
+grep -v ^X-Envelope- Mail/inbox/4
 ],
 [0],
 [X-Burst-Part: 1 1 01
 From: Sergey Poznyakoff <gray@example.net>
 To: root@example.com
 Subject: digest
-X-Envelope-Sender: gray@example.net
 
 Initial text.
 
@@ -57,13 +56,11 @@ X-Burst-Part: 1 2 02
 From: Puszcza hackers
 To: Sergey Poznyakoff <gray@gnu.org>
 Subject: Hello
-X-Envelope-Sender: GNU-Mailutils
 
 Greetings
 ---------
 How are you?
 X-Burst-Part: 1 3 00
-X-Envelope-Sender: GNU-MH
 
 Regards,
 Sergey
@@ -93,16 +90,15 @@ Sergey
 ])
 
 burst +inbox --length=7 1 || exit $?
-grep -v ^X-Envelope-Date Mail/inbox/2
-grep -v ^X-Envelope-Date Mail/inbox/3
-grep -v ^X-Envelope-Date Mail/inbox/4
+grep -v ^X-Envelope- Mail/inbox/2
+grep -v ^X-Envelope- Mail/inbox/3
+grep -v ^X-Envelope- Mail/inbox/4
 ],
 [0],
 [X-Burst-Part: 1 1 01
 From: Sergey Poznyakoff <gray@example.net>
 To: root@example.com
 Subject: digest
-X-Envelope-Sender: gray@example.net
 
 Initial text.
 
@@ -110,13 +106,11 @@ X-Burst-Part: 1 2 02
 From: Puszcza hackers
 To: Sergey Poznyakoff <gray@gnu.org>
 Subject: Hello
-X-Envelope-Sender: GNU-Mailutils
 
 Greetings
 -----
 How are you?
 X-Burst-Part: 1 3 00
-X-Envelope-Sender: GNU-MH
 
 Regards,
 Sergey