Split autogen.sh into autopull.sh and autogen.sh.
* autopull.sh: New file, based on autogen.sh.
* autogen.sh: Remove code that was moved to autopull.sh. Remove --skip-gnulib
option.
* HACKING: Mention autopull.sh.
* libtextstyle/autopull.sh: New file, based on libtextstyle/autogen.sh.
* libtextstyle/autogen.sh: Remove code that was moved to
libtextstyle/autopull.sh. Remove --skip-gnulib option.
* libtextstyle/HACKING: Mention autopull.sh.
* libtextstyle/m4/init-package-version.m4: Update comment.
