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* ssh-agent: lock this script with a mkdir style mutex

This script is a kind of singleton pattern and is not reentrant.
If several shells are oppened in a fast sequence, then several
independent ssh-agents would be created, which is not acceptable.
A mutex is required.
Signed-off-by: Nuno Goncalves <nunojpg@gmail.com>

* ssh-agent: only start agent if .ssh dir exists

To use the same profile system-wide, it might happen
that the .ssh directory does not exist
(typically $HOME/.ssh/). This would trigger a error.

Creating the directory would be a option, but it
usually will not make sense to do so because it means
the user doesn't have ssh keys or config.
Signed-off-by: Nuno Goncalves <nunojpg@gmail.com>

* ssh-agent: adds lazy option to disable key loading on start

Option is documented on updated README.md
Signed-off-by: Nuno Goncalves <nunojpg@gmail.com>

* ssh-agent: simplify agent-forwarding checking
Signed-off-by: Nuno Goncalves <nunojpg@gmail.com>
Co-authored-by: Robby Russell <robby@planetargon.com>