Add submodule-update-foreach.sh and a post-pull hook for it

Update submodules also after a "vcsh pull" command.

This requires a whole new script because vcsh does not call post-pull
hooks after pulling each repositories, it only calls the hook once after
pulling all of them.

See https://github.com/RichiH/vcsh/issues/213

When the issue gets fixed submodule-update-foreach.sh can be removed and
post-pull.00-submodule-init can link to submodule-update.sh