MINOR: enable IP_BIND_ADDRESS_NO_PORT on backend connections

Enable IP_BIND_ADDRESS_NO_PORT on backend connections when the source
address is specified without port or port ranges. This is supported
since Linux 4.2/libc 2.23.

If the kernel supports it but the libc doesn't, we can define it at
build time:
make [...] DEFINE=-DIP_BIND_ADDRESS_NO_PORT=24

For more informations about this feature, see Linux commit 90c337da
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 52e6cf4..dc43003 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -10936,6 +10936,9 @@
   total concurrent connections. The limit will then reach 64k connections per
   server.
 
+  Since Linux 4.2/libc 2.23 IP_BIND_ADDRESS_NO_PORT is set for connections
+  specifying the source address without port(s).
+
   Supported in default-server: No
 
 ssl