make .gnulib a submodule

This makes it so we record (via a git submodule)
a snapshot of whatever version of gnulib we're using,
and none of gnulib sources are in the libvirt repository.
The result is that we have as much reproducibility as when
we version-controlled imported copies of the gnulib sources,
but without the hassle of the manual process we used when
syncing with upstream.

Note that when you clone libvirt, you get only the libvirt
repository, but when you first run ./bootstrap, it clones
gnulib (at the SHA1 recorded via the submodule), creating
the .gnulib/ hierarchy.  Then, the bootstrap script runs
gnulib-tool to populate gnulib/ with the files that make
up the selected modules.

Put the following in your ~/.gitconfig file.
[alias]
  syncsub = submodule foreach git pull origin master

The update procedure is simple:
  git syncsub
  ...build & test...
  git commit -m 'gnulib: sync submodule to latest' .gnulib

* .gitmodules: New file.
* .gnulib: Initialize.
* bootstrap: Set up to use the new submodule.
Stop using --no-vc-files.
Don't remove .gitignore files.
Don't use or create .cvsignore.
Diagnose an invalid --gnulib-srcdir=DIR argument.
* build-aux/vc-list-files: Delete file, now pulled from gnulib.
* build-aux/useless-if-before-free: Likewise.
* po/POTFILES.in: Remove gnulib/lib/gai_strerror.c, since
it no longer contains translatable strings.
* gnulib/*: Remove gnulib/ hierarchy.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "gnulib"]
+	path = .gnulib
+	url = git://git.sv.gnu.org/gnulib.git