1.0.21.8: SERVE-EVENT error handling

1.0.21.8: SERVE-EVENT error handling

 * HANDLER-DESCRIPTORS-ERROR should not signal an error if there are
   no bad descriptors.

 * Check for EBADF and EINTR from select() explicitly, and inform the
   users of any other issues via SIMPLE-PERROR + CONTINUE restart. (We
   may be able to handle some others automatically as well, but let's
   figure out what occurs in the wild and why first.)