Do not start ssh/gpg agents on SSH login.

- it's faster
- we may already have a forwarded SSH agent
- they can always be started when needed
Clear the screen on logout (if $SHLVL == 1).
Rework ssh-gpg-agents.

- add support for -a (+ comma separated list of agents), -k (kill
  agents) and -r (restart agents)
- inherit ssh-agent if possible
- don't inherit gpg-agent for now