Upstream: simplified proxy_store and friends configuration code.

This changes internal API related to handling of the "store"
flag in ngx_http_upstream_conf_t.  Previously, a non-null value
of "store_lengths" was enough to enable store functionality with
custom path.  Now, the "store" flag is also required to be set.

No functional changes.
diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index c3105e2..323d03d 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -2628,10 +2628,8 @@
         ngx_conf_merge_value(conf->upstream.store,
                               prev->upstream.store, 0);
 
-        if (conf->upstream.store_lengths == NULL) {
-            conf->upstream.store_lengths = prev->upstream.store_lengths;
-            conf->upstream.store_values = prev->upstream.store_values;
-        }
+        conf->upstream.store_lengths = prev->upstream.store_lengths;
+        conf->upstream.store_values = prev->upstream.store_values;
     }
 
     ngx_conf_merge_uint_value(conf->upstream.store_access,
@@ -3794,9 +3792,7 @@
     ngx_str_t                  *value;
     ngx_http_script_compile_t   sc;
 
-    if (plcf->upstream.store != NGX_CONF_UNSET
-        || plcf->upstream.store_lengths)
-    {
+    if (plcf->upstream.store != NGX_CONF_UNSET) {
         return "is duplicate";
     }
 
@@ -3817,8 +3813,9 @@
 
 #endif
 
+    plcf->upstream.store = 1;
+
     if (ngx_strcmp(value[1].data, "on") == 0) {
-        plcf->upstream.store = 1;
         return NGX_CONF_OK;
     }
 
@@ -3863,7 +3860,7 @@
         return NGX_CONF_OK;
     }
 
-    if (plcf->upstream.store > 0 || plcf->upstream.store_lengths) {
+    if (plcf->upstream.store > 0) {
         return "is incompatible with \"proxy_store\"";
     }