On system where my dotfiles are only partially installed (e.g. only
the shell) for the environment variables defined in the zprofile to take
affect, every terminal instance needs to execute zsh a login-shell.
This lead to numerous instances of the ssh-agent that were not killed
when exiting the shell. As I still need the correct env-vars like
SSH_AGENT_PID in every shell I cannot just pgrep and execute the agent
respectively.
This adds a zlogout that kills the agent spawned in the current shell.
It also stops `exec`ing `startx` as then the zlogout is not read.
