Moving sys:capture-cont to call/cc style API.

* unwind.c (revive_cont): Don't wrap cons cell around
passed arg; just pass it directly. We don't need
that convention any more.

* capture_cont: Take functional argument. Pass the captured
continuation to the function. If the function returns,
return whatever it returned. When resuming, return the
continuation argument.
(uw_capture_cont): Take functional second argument
and pass to capture_cont. Context form becomes third
argument.
(uw_late_init): Update registration of sys:capture-cont
to three arguments, two required.

* unwind.h (uw_capture_cont): Declaration updated.

* share/txr/stdlib/yield.tl (sys:yield-impl): Not
needed any more; all this was doing was implementing
a call/cc style interface around sys:capture-cont
which can now be used directly.
(yield-from): Use sys:capture-cont directly.
(suspend): Simplified to the point of triviality
with new sys:capture-cont.

* txr.1: Documented.

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