* throw.h (scm_c_catch, scm_c_with_throw_handler,

* throw.h (scm_c_catch, scm_c_with_throw_handler,
scm_catch_with_pre_unwind_handler, scm_with_throw_handler): New.

* throw.c (SCM_JBPREUNWIND, SCM_SETJBPREUNWIND): New.
(struct pre_unwind_data): New, replaces struct lazy_catch.
(scm_c_catch): New, replaces scm_internal_catch as the primary
catch API for C code; adds pre-unwind handler support.
(scm_internal_catch): Now just a wrapper for scm_c_catch, for back
compatibility.
(tc16_pre_unwind_data, pre_unwind_data_print,
make_pre_unwind_data, SCM_PRE_UNWIND_DATA_P): Renamed from
"lazy_catch" equivalents.
(scm_c_with_throw_handler): New, replaces scm_internal_lazy_catch
as the primary C API for a "lazy" catch.
(scm_internal_lazy_catch): Now just a wrapper for
scm_c_with_throw_handler, for back compatibility.
(scm_catch_with_pre_unwind_handler): Renamed from scm_catch; adds
pre-unwind handler support.
(scm_catch): Now just a wrapper for
scm_catch_with_pre_unwind_handler, for back compatibility.
(scm_with_throw_handler): New.
(scm_lazy_catch): Update comment to say that the handler can
return, and what happens if it does.
(toggle_pre_unwind_running): New.
(scm_ithrow): When identifying the throw target, take running
flags into account.  In general, change naming of things from
"lazy_catch" to "pre_unwind".  When throwing to a throw handler,
don't unwind the dynamic context first.  Add dynwind framing to
manage the running flag of a throw handler.  If a lazy catch or
throw handler returns, rethrow the same exception again.  Add
pre-unwind support to the normal catch case (SCM_JMPBUFP).

* root.c (scm_internal_cwdr): Add NULL args to
scm_i_with_continuation_barrier call.

* dynwind.c: Change comment mentioning lazy-catch to mention
pre-unwind data and throw handler also.

* continuations.h (scm_i_with_continuation_barrier): Add
pre-unwind handler args.

* continuations.c (scm_i_with_continuation_barrier): Add
pre-unwind handler args, and pass on to scm_c_catch (changed from
scm_internal_catch).
(c_handler): Remove scm_handle_by_message_noexit call.
(scm_c_with_continuation_barrier): Call
scm_i_with_continuation_barrier with scm_handle_by_message_noexit
as the pre-unwind handler.
(scm_handler): Remove scm_handle_by_message_noexit call.
(s_scm_with_continuation_barrier): Call
scm_i_with_continuation_barrier with scm_handle_by_message_noexit
as the pre-unwind handler.