Not every "session" channel in SSH allocates a pty at all, of course,
and so I'll need a way to run a subprocess without doing so. The
simplest approach seems to be to expand uxpty's remit so that the pty
is optional: now it can open either a pty or a set of pipes for
stdin/out/err, according to an option provided to pty_backend_create.
(It amuses me that without this option I'd have an SSH server which is
incapable of _not_ honouring the "pty-req" channel request. That's
normally the easy part!)
This breaks the previous one-to-one coupling between pty backend
instances and file descriptors passed to uxsel, which I was using to
look up the Pty structure in a tree234 indexed by fd when an uxsel
notification came back. So now each Pty structure contains a
collection of subobjects of a new type PtyFd, and _those_ are what's
stored in the fd-indexed tree.
Another awkward part is that uxsel_set is not incremental: the rwx
flags you pass to it completely supersede the previous set for that
file descriptor, so I had to set up the logic that decides whether
we're trying to read or write each fd in a way that can cope equally
well with the fd aliasing another one (if it's the pty master) or not
(if there are three completely separate pipes).
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