[main] make main() no-op when called again

Fix #390

Suggested by @davidcarlisle.

src/luaotfload-main.lua

@@ -246,8 +246,18 @@ local install_loaders = function ()
     return loaders
 end
 
+local luaotfload_initialized = false --- prevent multiple invocations
+
 luaotfload.main = function ()
 
+    if luaotfload_initialized then
+        logreport ("log", 0, "load",
+                   "Luaotfload initialization requested but is already \z
+                   loaded, ignoring.")
+        return
+    end
+    luaotfload_initialized = true
+
     luaotfload.loaders = install_loaders ()
     local loaders    = luaotfload.loaders
     local loadmodule = loaders.luaotfload