[Minor] Make lua_ffi more flexible to use

lualib/lua_ffi/init.lua

@@ -34,8 +34,7 @@ else
   local ret,result_or_err = pcall(require, 'ffi')
 
   if not ret then
-    io.stderr:write('FFI support is required: please use LuaJIT or install lua-ffi')
-    os.exit(1)
+    return {}
   end
 
   ffi = result_or_err
@@ -51,4 +50,9 @@ exports.common = require "lua_ffi/common"
 exports.dkim = require "lua_ffi/dkim"
 exports.spf = require "lua_ffi/spf"
 
+for k,v in pairs(ffi) do
+  -- Preserve all stuff to use lua_ffi as ffi itself
+  exports[k] = v
+end
+
 return exports

lualib/rspamadm/mime.lua

@@ -754,6 +754,11 @@ local function sign_handler(opts)
 
   local lua_dkim = require("lua_ffi").dkim
 
+  if not lua_dkim then
+    io.stderr:write('FFI support is required: please use LuaJIT or install lua-ffi')
+    os.exit(1)
+  end
+
   local sign_key
   if rspamd_util.file_exists(opts.key) then
     sign_key = lua_dkim.load_sign_key(opts.key, 'file')