[MEDIUM] add support for source interface binding

Specifying "interface <name>" after the "source" statement allows
one to bind to a specific interface for proxy<->server traffic.

This makes it possible to use multiple links to reach multiple
servers, and to force traffic to pass via an interface different
from the one the system would have chosen based on the routing
table.

doc/configuration.txt

@@ -2839,6 +2839,7 @@ server <name> <address>[:port] [param*]
 
 
 source <addr>[:<port>] [usesrc { <addr2>[:<port2>] | client | clientip } ]
+source <addr>[:<port>] [interface <name>]
   Set the source address for outgoing connections
   May be used in sections :   defaults | frontend | listen | backend
                                  yes   |    no    |   yes  |   yes
@@ -2864,6 +2865,13 @@ source <addr>[:<port>] [usesrc { <addr2>[:<port2>] | client | clientip } ]
               The default value of zero means the system will select a free
               port.
 
+    <name>    is an optional interface name to which to bind to for outgoing
+              traffic. On systems supporting this features (currently, only
+              Linux), this allows one to bind all traffic to the server to
+              this interface even if it is not the one the system would select
+              based on routing tables. This should be used with extreme care.
+              Note that using this option requires root privileges.
+
   The "source" keyword is useful in complex environments where a specific
   address only is allowed to connect to the servers. It may be needed when a
   private address must be used through a public gateway for instance, and it is