Fixed and improved the "*_bind" directives of proxying modules.

The "proxy_bind", "fastcgi_bind", "uwsgi_bind", "scgi_bind" and
"memcached_bind" directives are now inherited; inherited value
can be reset by the "off" parameter.  Duplicate directives are
now detected.  Parameter value can now contain variables.
diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index 12a54c7..a826573 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -2369,6 +2369,8 @@
     conf->upstream.buffering = NGX_CONF_UNSET;
     conf->upstream.ignore_client_abort = NGX_CONF_UNSET;
 
+    conf->upstream.local = NGX_CONF_UNSET_PTR;
+
     conf->upstream.connect_timeout = NGX_CONF_UNSET_MSEC;
     conf->upstream.send_timeout = NGX_CONF_UNSET_MSEC;
     conf->upstream.read_timeout = NGX_CONF_UNSET_MSEC;
@@ -2453,6 +2455,9 @@
     ngx_conf_merge_value(conf->upstream.ignore_client_abort,
                               prev->upstream.ignore_client_abort, 0);
 
+    ngx_conf_merge_ptr_value(conf->upstream.local,
+                              prev->upstream.local, NULL);
+
     ngx_conf_merge_msec_value(conf->upstream.connect_timeout,
                               prev->upstream.connect_timeout, 60000);