Include gnulib and paxutils as submodules.

* .gitmodules: New file.
* README-alpha: Update.
* README-hacking: Update.
* bootstrap: Install slightly modified version from the gnulib repo.
* bootstrap.conf: Add paxutils-related stuff.
* .gitignore: Update.
* doc/.gitignore: Update.
* po/.gitignore: Update.
* gnu/Makefile.am

doc/.gitignore

@@ -1,7 +1,6 @@
 genfile.texi
 header.texi
 manual
-parse-datetime.texi
 stamp-vti
 tar.aux
 tar.cp
@@ -24,3 +23,4 @@ tar.toc
 tar.tp
 tar.vr
 version.texi
+/parse-datetime.texi