Provide a dotfiles update script

[KellerFuchs]

We should provide a script that pulls dotfiles updates:

1. Create a scratch GNUPGHOME with the right configuration
2. Run git -C ~/.dotfiles fetch origin master
3. git -C ~/.dotfiles verify-commit origin/master, abort if improperly signed.
4. Merge the remote, abort and cleanup if there is a conflict
5. Update submodules to the commits specified by the base repo.