with-continuation-barrier calls exit(3) _after_ unwinding

with-continuation-barrier calls exit(3) _after_ unwinding

* libguile/continuations.c (scm_handler, c_handler)
  (scm_c_with_continuation_barrier, scm_with_continuation_barrier):
  Instead of calling scm_handle_by_message_noexit in the pre-unwind
  handler, roll our own exception printing in the pre-unwind, and do to
  exit()-on-quit in the post-unwind handler.  This lets the stack unwind
  at exit-time so that pending dynwinds run.

* test-suite/tests/continuations.test ("continuations"): Add a test.