signal meanings: rotate_logs_handler() was changing the SIGHUP handler
to rotate the logs!  zdaemon did not do anything wrong. :-)
Fixed by *removing* the signal() call inside the handler -- this is
not needed in modern Unixes.

Also changed setup_signals(): there was a typo in the name SIFXFSZ(!);
and made all the signal conditional on whether the signal module has
that particular signal (e.g. on Windows, a signal module exists, but
it only defines a few signals).