* nick/ssh/fix-process-leak/OTP-8807:
  Fix race condition when terminating a connection.
  fix process leak in ssh_system_sup (dynamicaly created childs where not cleaned up)

This was caused by a race condition between two processes.

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.

    OTP-8735  SSH in some cases generated a crash report when a channel
              was closed in a normal way.

    OTP-8714  SSH in some cases terminated channels with reason normal
              when it should have been shutdown.

    OTP-8550  The function ssh:connect/4 was not exported.

    OTP-8644  Aligned error message with used version (SSH_FX_FAILURE
              vs SSH_FX_NOT_A_DIRECTORY, the latter introduced in version
              6).

    OTP-8645  Resolved race condition when another connection is started
              before a channel is opened in the first connection.

    OTP-8535  The configuration parameter ip_v6_disabled is now available,
              which makes it possible for the user to alter the IP version
              SSH shall use.

    OTP-8534  The ssh_connection:send operation now accepts infinity as
              timeout.

    OTP-8524  The connection handler now include stack traces when a channel
              message is not handled correctly.