better handling for exceptions in close-port and port finalizers

better handling for exceptions in close-port and port finalizers

* libguile/fports.c (close_the_fd, fport_close): Arrange to always close
  the fd, even if the flush procedure throws an exception.  Perhaps the
  port machinery should do this for us, though.  Don't wrap the close
  call in SCM_SYSCALL, EINTR leaves the fd in an unspecified state.
  Don't bother freeing buffers, the collector will handle that; simply
  drop references via scm_port_non_buffer.

* libguile/ports.c (do_free, finalize_port): Catch exceptions caused by
  the free procedure.  Don't bother setting the stream to 0 at all.
  (scm_close_port): Ensure that exceptions thrown by the "close"
  procedure don't prevent the port from being marked as closed.