Update submodules.

1) Specify master branch for each submodule, since for some reason they
end up with a detached HEAD.

2) Checkout the master branch using git submodule foreach git checkout
master.

3) Pull in the latest upstream changes using git submodule foreach git
pull.

4) Specify ignore = dirty for each submodule in .gitmodules to prevent
git status from notifying of submodule changes, only changes in the
superproject will be taken into account.