Previously we waited for all opened file descriptors to the terminal
to be closed. This caused problem when e.g. running "sleep 900 &"
and then exiting the shell, with sleep keeping the session alive and
had to be killed manually (killing the process group did not help -
the shell had already exited and was in zombie state). This is also
what most other terminal emulators do.
Relatedly, switch to sending SIGKILL to force quit a session instead
of SIGHUP, since SIGHUP can be ignored.
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