[MINOR] redirect: add support for the "drop-query" option

If "drop-query" is present on a "redirect" line using the "prefix" mode,
then the returned Location header will be the request URI without the
query-string. This may be used on some login/logout pages, or when it
must be decided to redirect the user to a non-secure server.

(cherry-picked from commit f2d361ccd73aa16538ce767c766362dd8f0a88fd)
diff --git a/include/types/proxy.h b/include/types/proxy.h
index 721d7c0..a7d2c40 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -266,6 +266,7 @@
 	int rdr_len;
 	char *rdr_str;
 	int code;
+	unsigned int flags;
 };
 
 extern struct proxy *proxy;