gluon-core: add util gluon-show-site to print merged site config

This should not convert JSON to a Lua table and back, as this loses the
distinction between arrays and objects, but as our site.conf is defined in
Lua anyways (for now), this can be fixed in a later revision.

[Matthias Schiffer: rename to gluon-show-site, rebase]

package/gluon-core/luasrc/usr/bin/gluon-show-site

+#!/usr/bin/lua
+
+local json = require 'jsonc'
+local site = require 'gluon.site'
+
+print(json.stringify(site(), true))