Introducing thread-safe Tramp

[Michael Albinus]

Hi,

I have created a new branch feature/tramp-thread-safe. The major change
is making Tramp thread-safe, that means, several basic file operations
can run concurrently now.

Being a library, Tramp does not create new threads on its own. This must
be performed by the callers.

I have changed the `find-file' family of commands to raise their
underlying file operations asynchronously if indicated. Every visiting
of a file happens in its own thread then. If wildcards are used, one
thread per involved file is created.

In order to enable this, the respective command must be called with a
prefix argument, like "C-u C-x C-f ...".  Emacs will remain responsive,
you can continue with whatever operation. This will be best seen with
remote files, because they need more time to load into a buffer. Emacs
is not as responsive yet as I hoped to achieve; further optimization is
needed.

As second change, I have modified `vc-refresh-state' to run always in
its own thread. This is because I have observed, that for git
repositories, loading a remote file is often much faster than the
following update of its state in the modeline. For the time being this
behaviour is hard-coded, I haven't seen any drawback for this even for
local files. But we could trigger it also via a user option.

@Dmitry?

Please give it as much testing as possible. I cannot promise Emacs won't
crash ever with these changes, so it is not time for production
systems. But all bug reports and feedbacks will help to improve this new
feature.

And, if there are not too serious complaints, I will merge it into
master after a while :-) You are warned!

Best regards, Michael.