Move user eval function to be located on Lua registry.

Today, Redis wrap the user Lua code with a Lua function.
For example, assuming the user code is:

```
return redis.call('ping')
```

The actual code that would have sent to the Lua interpreter was:

```
f_b3a02c833904802db9c34a3cf1292eee3246df3c() return redis.call('ping') end
```

The wraped code would have been saved on the global dictionary with the
following name: `f_<script sha>` (in our example `f_b3a02c833904802db9c34a3cf1292eee3246df3c`).

This approach allows one user to easily override the implementation a another user code, example:

```
f_b3a02c833904802db9c34a3cf1292eee3246df3c = function() return 'hacked' end
```

Running the above code will cause `evalsha b3a02c833904802db9c34a3cf1292eee3246df3c 0` to return
hacked although it should have returned `pong`.

Another disadventage is that Redis basically runs code on the loading (compiling) phase without been
aware of it. User can do code injection like this:

```
return 1 end <run code on compling phase> function() return 1
```

The wraped code will look like this and the entire `<run code on compling phase>` block will run outside
of eval or evalsha context:

```
f_<sha>() return 1 end <run code on compling phase> function() return 1 end
```

1 files changed, 2 insertions, 2 deletions

diff --git a/src/script_lua.c b/src/script_lua.c
index 9a08a7e47..4e1f17649 100644
--- a/src/script_lua.c
+++ b/src/script_lua.c
@@ -1144,7 +1144,7 @@ sds luaGetStringSds(lua_State *lua, int index) {
  * On Legacy Lua (eval) we need to check 'w ~= \"main\"' otherwise we will not be able
  * to create the global 'function <sha> ()' variable. On Functions Lua engine we do not use
  * this trick so it's not needed. */
-void luaEnableGlobalsProtection(lua_State *lua, int is_eval) {
+void luaEnableGlobalsProtection(lua_State *lua) {
     char *s[32];
     sds code = sdsempty();
     int j = 0;
@@ -1157,7 +1157,7 @@ void luaEnableGlobalsProtection(lua_State *lua, int is_eval) {
     s[j++]="mt.__newindex = function (t, n, v)\n";
     s[j++]="  if dbg.getinfo(2) then\n";
     s[j++]="    local w = dbg.getinfo(2, \"S\").what\n";
-    s[j++]=     is_eval ? "    if w ~= \"main\" and w ~= \"C\" then\n" : "    if w ~= \"C\" then\n";
+    s[j++]="    if w ~= \"C\" then\n";
     s[j++]="      error(\"Script attempted to create global variable '\"..tostring(n)..\"'\", 2)\n";
     s[j++]="    end\n";
     s[j++]="  end\n";