nginx: refactor and add mainline version

Upstream calls the unstable version mainline.

pkgs/servers/http/nginx/mainline.upstream

@@ -0,0 +1,8 @@
+url http://nginx.org/en/download.html
+version_link '.*-([0-9]+[.][0-9]+([.][0-9]+)*)[.]tar[.][a-z0-9]*$' 
+
+do_overwrite() {
+  ensure_hash
+  set_var_value version "$CURRENT_VERSION"
+  set_var_value sha256 "$CURRENT_HASH"
+}