fix process leak in ssh_system_sup (dynamicaly created childs where not cleaned up)

[Stefan Grundmann]

The ssh_system_sup supervisor supervises one ssh_subsystem_sup process for
every client connection. There was no functionality to free resources
(terminate_child/ delete_child) when a client connection was closed.
Which lead to one ssh_subsystem_sup and one ssh_channel_sup process left over.

This commit adds ssh_system_sup:stop_subsystem/2 and code that calls it
in ssh_connection_manager:terminate/2.

git fetch git://github.com/sg2342/otp.git fix_ssh_system_sup_proc_leak

best regards

Stefan Grundmann