Assume gnulib is checked out from Git, not CVS

Assume gnulib is checked out from Git, not CVS

In fact, access to the gnulib repository through CVS has been disabled,
or more precisely, got broken and was never restored; see:
<http://lists.gnu.org/archive/html/bug-gnulib/2013-05/msg00008.html>

Note that support for CVS is not removed completely and unthinkingly
by this change: only support for CVS checkouts of gnulib itself is
removed.  For example, the 'bootstrap' script still cater to .cvsingore
files and CVS directories, for the benefit of those poor gnulib clients
still stuck with CVS.  Ditto for the 'gnulib-tool' script itself.

* gnulib-tool: Simplify accordingly.
* posix-modules: Likewise.
* MODULES.html.sh: Likewise.
* doc/gnulib.texi: No longer mention the decommissioned CVS gnulib
repository.
* doc/gnulib-intro.texi: Likewise.
* doc/gnulib-readme.texi: Likewise.
* doc/gnulib-tool.texi: In the examples and explanations, refer to a
sample '.gitignore' file rather than a sample '.cvsignore'.
* NEWS: Update.
* m4/extensions.m4: While at it, remove a comment mistakenly referring
to "CVS Autoconf" rather than "git Autoconf".

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Acked-by: Paul Eggert <eggert@cs.ucla.edu>