I/O [NT]: Keep track of processes using ConPTY slaves.

Due to the way that ptys are implemented on NT, the master side
read pipe will not receive an EOF when all clients have terminated
(since the ConPTY process is still running).

We now keep track of the processes and kill the ConPTY when all
the processes using the slave side are gone. Note that this is
NOT 100% (or even 80%) safe. Indirect processes may still be
running, or processes may terminate after we have already entered
a blocking read().

A feature request has been registered with the Windows Terminal
team (https://github.com/microsoft/terminal/issues/4564).