When in an SSH session, try to connect to a forwarded agent first. If
that fails, only then check if an SSH agent is already running on the
machine.
This fixes a situation where a .ssh/environment file left behind from
a dead agent is checked first, ignoring the forwarded agent and
starting a new agent on the machine.
Utility functions to start your ssh agent when using fish shell.
You will only need to run ssh-add and type your password once,
after the running ssh_agent should do the work for you.