Change dynwind flag enums to #defines, for greater portability

Change dynwind flag enums to #defines, for greater portability

Thanks to Inge Gutheil for raising this problem.

* doc/ref/api-control.texi (Dynamic Wind): Remove doc for
  scm_t_dynwind_flags, and change uses of scm_t_dynwind_flags to int.
  Mention possible flags inside doc for scm_dynwind_begin instead.
  Similarly for scm_t_wind_flags.  In this case the doc on possible
  flags is moved to after the four functions that can use them.

* libguile/dynwind.c (scm_dynwind_begin): Change scm_t_dynwind_flags
  to int.
  (scm_dynwind_unwind_handler): Change scm_t_wind_flags to int.
  (scm_dynwind_rewind_handler): Ditto.
  (scm_dynwind_unwind_handler_with_scm): Ditto.
  (scm_dynwind_rewind_handler_with_scm): Ditto.

* libguile/dynwind.h (SCM_F_DYNWIND_REWINDABLE): Use #define instead
  of typedef enum.
  (SCM_F_WIND_EXPLICITLY): Ditto.
  (function declarations): Change scm_t_dynwind_flags and
  scm_t_wind_flags to int.

* test-suite/standalone/test-unwind.c (check_cont_body): Change
  scm_t_dynwind_flags to int.