Add functions to 'code' to allow loading of multiple modules
at once.

code:atomic_load(Modules) will load all modules at once, or fail
having loaded none of them. Since we cannot guarantee the atomicity if
there are modules with -on_load functions, the list of modules must
not contain any modules with an -on_load function.

Also, to make it possible to put an application into an inactive state
for as short time as possible, also add code:prepare_loading/1 and
code:finish_loading/1. They are used like this:

  {ok,Prepared} = code:prepare_loading(Modules)
    .
    .
    .
  ok = code:finish_loading(Prepared)

code:ensure_modules_loaded/1 is useful as a pure optimization to
ensure that modules that will be needed soon have indeed been
loaded. It will not reload modules that have already been loaded and
it *will* accept modules that have an on_load function. Therefore, it
does not make sense to give any atomicity guarantees.

I did consider overloading the existing code:ensure_loaded/1
function, but rejected it because the return value is very
different. Having different forms of return values depending
on the types of arguments is confusing.

Since no test suites includede test_server.hrl, there is no need
to have test_server in the include path or code path.

Smoke tests are meant to verify that a build of erlang has been successfull.