syslog-ng: detect disabled IPv6 on loopback and fallback to IPv4

Binding in default to IPv6 is preferable but it can be disabled in
kernel and that prevents syslog-ng to start. This setup should not be
that common but syslog is very important service and should survive
that.

This introduces new plugin defining source generator
`network_localhost`. This is used instead of original network source.

Signed-off-by: Karel Kočí <cynerd@email.cz>

admin/syslog-ng/files/syslog-ng.conf

@@ -26,7 +26,7 @@ source src {
 };
 
 source net {
-	network(ip("::1") port(514) transport(udp) ip-protocol(6));
+	network_localhost();
 };
 
 source s_network {