ffi: a measure of safety for callbacks.

We don't want, by default, for callbacks to capture delimited
continuations across foreign code, or perpetrate non-local
transfers across foreign code. Here, we take an approach
similar for what was done in ftw_wrap.

* ffi.c (s_exit_point): New global variable with internal
linkage.
(ffi_call_wrap): If s_exit_point isn't nil, then it means that
the callback intercepted a nonlocal transfer and stored its
exit point. We resume the transfer to that exit point instead
of returning normally.
(ffi_closure_dispatch_safe): New static function.
(ffi_make_closure): Support a new argument which indicates
whether to make a closure which uses
ffi_closure_dispatch_safe, or ffi_closure_dispatch.
(ffi_init): Update registration of ffi-make-closure intrinsic.

* ffi.h (ffi_make_closure): Declaration updated.

* share/txr/stdlib/ffi.tl (sys:deffi-cb-expander): New
function.
(deffi-cb): Macro internals replaced by call to new
function.
(deffi-cb-safe): New macro.

* txr.1: Documentation of ffi-make-closure updated.
New deffi-cb-unsafe macro documented.

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