Improve on sharing my config with others

- Rename gitconfig.local to gitconfig.extra, since it reflects better what's
  intended.
- Remove user.name and user.email from the gitconfig.extra file, since it is
  supposed to be in the main ~/.gitconfig, so others can set it with "git config
  --global" to theirs.
- Change the setup-unix target on the makefile, and split it in two. One is for
  the vim configuration alone, and the main one is for everything else. This
  eases the problem that the vim config requires lots of submodules.
Create .ssh dir before symlinking .ssh/config
Check the presence of ~/.ssh to not let mkdir fail