Clone ssh-agent's subprocess option

[cuviper]

 If a commandline is given, this is executed as a subprocess of the agent.
 When the command dies, so does the agent.

[cuviper]

Support running a subprocess

If extra command-line arguments are found then they are used as a
command for a subprocess, and ssh-pageant will run for the life of that
process instead of daemonizing.

Closed by 9ff8fd3