nanny: reset worker on enable(), disable()

rearming is not enough as often reseting internal fsm
is mandatory.
nanny: arm nanny only if ni_config_use_nanny() is true

Nanny is still allowed to start in order not to freak out
systemd, but does not register any signals nor performs
any actions.
Merge pull request #396 from pwieczorkiewicz/nanny-reset

nanny: reset workers on enable/disable and do not arm nanny it is not in use (bnc#891045)