Detect disconnected SSH sessions

[daonb]

Describe the bug
When coming bug from a sleep, T7 assumes connected ssh sessions are still on.
If the active gate was disconnected the screen freeze

To Reproduce
Steps to reproduce the behavior:

1. Launch app
2. Connect to a server using ssh
3. put the iPad to sleep
4. count till down
5. wake up
6. See frozen T7

Additional context
A possible fix is to send an SSH ping whenever the app is back from sleep and
validate the connection is still on.
The pinger can be based on ssh2_echo.c

The code sends a 'cat' command, and then writes a lot of data to it only to
check that reading the returned data sums up to the same amount.

probably best to do it in a new dedicated channel.