Partial revert of 0a97e68c48 (Special handling of signals for cf-key)

In that commit, the signal handler for SIGPIPE was removed. SIGPIPE
can easily happen when you pipe your input into for example grep, so
we need to restore this. We ignore the signals so any output after the
signal will just go to the void.
examples/container_key_iteration.cf: add a container key iteration example