MEDIUM: config: make str2listener() not accept datagram sockets anymore

str2listener() was temporarily hacked to support datagram sockets for
the log-forward listeners. This has has an undesirable side effect that
"bind udp@1.2.3.4:5555" was silently accepted as TCP for a bind line.

We don't need this hack anymore since the only user (log-forward) now
relies on str2receiver(). Now such an address will properly be rejected.
diff --git a/tests/ports.cfg b/tests/ports.cfg
index 7210c3c..5779348 100644
--- a/tests/ports.cfg
+++ b/tests/ports.cfg
@@ -14,6 +14,7 @@
 	 bind :::11003-11010
 	 bind :::+11011                      # port offset not permitted here in ':::+11011'
 	 bind :::-11012                      # port offset not permitted here in ':::-11012'
+	 bind udp@:11013                     # dgram-type socket not acceptable in 'udp@:11013'
 
 frontend f2
 	 bind :::0                           # invalid port '0'