Fix the case where one goes from zero to more than zero items enabled
in /etc/ttys. Before this fix, once the count of active services
reaches 0, one could never restart any more without a reboot.

Steve Passe did the leg work on this patch. After he found the fix,
we discovered that an identical fix had been made to NetBSD.

Approved by: re@ <scottl>
Approval tool: peril sensitive sunglasses