SI-10007 sys.process thread sync

A previous change to replace `SyncVar.set` with `SyncVar.put`
breaks things.

This commit tweaks the thread synchronizing in `sys.process`
to actually use `SyncVar` to sync and pass a var.

Joining the thread about to exit is superfluous.

A result is put exactly once, and consumers use
non-destructive `get`.

Note that as usual, avoid kicking off threads in a static
context, since class loading cycles are somewhat dicier
with 2.12 lambdas. In particular, REPL is a static context
by default.

SI-10007 Clarify deprecation message

The message on `set` was self-fulfilling, as it didn't
hint that `put` has different semantics.

So explain why `put` helps avoid errors instead of
creating them.

SI-10007 Always set exit value

Always put a value to exit code, defaulting to None.

Also clean up around tuple change to unfortunately
named Future.apply. Very hard to follow those types.

Date command pollutes output, so tweak test.

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