This commit implements the ssh exec channel request. It also handles
some shortcommings on the interactive channel like exit and concurrent
session establishments.
Pipes into the channel do not work yet. E.g.:
echo foobar | ssh noux@localhost -p 5555 "cat > /rw/test.txt"
The issue described with FIXME in Ssh::Server::incoming_connection()
could not be reproduced and might have been fixed with the improved
file descriptor handling.
Fixes#3401
