ffi: Follow example of C_call_scheme; eliminate aborts.

ffi: Follow example of C_call_scheme; eliminate aborts.

Keep callouts simple, fast.  Make callbacks save/restore machine state
(last_return_code, mainly) and use RC_END_OF_COMPUTATION instead of a
special primitive (return-to-c), like C_call_scheme.  Punt the
pop_return_p parameter of Interpret; instead, leave #f in EXP and
Re_Enter_Interpreter, also like C_call_scheme.  Add re_enter_scheme to
pop machine state pushed by abort_to_c (used when state cannot be
saved locally, as in the glib plugin's run_glib).

Add a gc-flip to the test callback.