queue: Make DSN tidier, especially in handling multi-line errors

This patch contains some changes to generate tidier DSNs, which should
make them slightly more readable.

In particular, it also makes it able to handle multi-line errors much
better than before.

test/t-05-null_address/expected_dsr

@@ -23,6 +23,7 @@ Delivery of your message to the following recipient(s) failed permanently:
 
   - fail@testserver
 
+
 Technical details:
 - "false" (PIPE) failed permanently with error:
     exit status 1