[MINOR] add X-Original-To: header

I have attached a patch which will add on every http request a new
header 'X-Original-To'. If you have HAProxy running in transparent mode
with a big number of SQUID servers behind it, it is very nice to have
the original destination ip as a common header to make decisions based
on it.

The whole thing is configurable with a new option 'originalto'. I have
updated the sourcecode as well as the documentation. The 'haproxy-en.txt'
and 'haproxy-fr.txt' files are untouched, due to lack of my french
language knowledge. ;)

Also the patch adds this header for IPv4 only. I haven't any IPv6 test
environment running here and don't know if getsockopt() with SO_ORIGINAL_DST
will work on IPv6. If someone knows it and wants to test it I can modify
the diff. Feel free to ask me questions or things which should be changed. :)

--Maik
diff --git a/include/common/defaults.h b/include/common/defaults.h
index fb8d188..07cb75f 100644
--- a/include/common/defaults.h
+++ b/include/common/defaults.h
@@ -130,6 +130,9 @@
 // X-Forwarded-For header default
 #define DEF_XFORWARDFOR_HDR	"X-Forwarded-For"
 
+// X-Original-To header default
+#define DEF_XORIGINALTO_HDR	"X-Original-To"
+
 /* Default connections limit.
  *
  * A system limit can be enforced at build time in order to avoid using haproxy