Racket: better help commands

For some reason that i don't fully understand, evaluating a function
in the racket/base namespace first thing after loading errortrace
breaks the help macro (!).  This patches provides a workaround by
actually invoking help first thing when Geiser starts, with alibi that
it serves to preload the help index (in a separate thread).

While i was at it, i improved the message printed in the minibuffer
when no help is found.
Racket: pre-loading help also in remote REPLs
Racket: fixing help when REPL is in a module

The evaluation of the help form must happen in a good enough
namespace.
Racket: ensuring the first C-c C-d C-i works

... by the obvious device of waiting for the thread building the index
to finish.
Racket: correct detection of manual lookup failures

Still a tad messy, because we are always forcing a retort-syntax error
and checking its ouput, but good enough for now.
NEWS entry since Diogo confirmed things work again
Company mode screencast seems gone for good

And Dmitry is now maintaining it.  Links updated, issue #3 fixed.
Remainin reference to company's screencast gone.

Fixes #3 for real