New way of handling exceptions without unwinding.

* eval.c (handler_bind_s): New symbol variable.
(op_handler_bind): New static function.
(do_expand): Traverse handler-bind forms.
(eval_init): Initialize handler_bind_s variable and register handler-bind
operator.

* lisplib.c (except_set_entries, except_instantiate): New functions.
(lisplib_init): Register new functions in dl_table.

* parser.c (intr_s): New symbol variable.
(repl_intr): Throw exception of type intr, rather than error.
This way we can interrupt accidental exception handling loops involving
exceptions derived from error.
(parse_init): Initialize intr_s.

* share/txr/stdlib/except.tl: New file, defines handle macro.

* unwind.c (uw_push_handler): New function.
(invoke_handler): New static function.
(uw_throw): Search loop looks for and processes handlers in addition to
catches.

* unwind.h (uw_frtype_t): New enum member, UW_HANDLE.
(struct uw_catch): Move member visible so it
is in the same position as in struct uw_handler.
(struct uw_handler): New struct type.
(union uw_frame): New member ha of type struct uw_handler.
(uw_push_handler): Function declared.

* txr.1: Added introductory paragraphs to Exception Handling section.
Documented handler-bind and handle. Some minor errors corrected.

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