Problem: broke signal handling for some apps

Commit ce664b9 removed the ability for apps to override CZMQ's
handling of SIGINT/SIGTERM, which is a breaking change with no
workaround.

Solution: re-instate zsys_handler_set(NULL) to mean 'use an
external signal handler'. Current code is hopefully cleaner and
more obvious now, and this call can be used at any point to
disable CZMQ's internal signal handling.