This fixes a behaviour change made by 44ac1020.
That commit make nm-online to wait for NM finishing startup instead of waiting
for a real connection. So for NetworkManager fully initialized, but
disconnected nm-online would return 0.

$ nmcli -f RUNNING,STATE,STARTUP,CONNECTIVITY gen status
RUNNING  STATE         STARTUP  CONNECTIVITY
running  disconnected  started  none

Revert back to the original behaviour of waiting for a connection. And
introduce a new option '--wait-for-startup' waiting for NetworkManager
finishing its startup, which is useful in some cases, like
NetworkManager-wait-online.service.

https://bugzilla.redhat.com/show_bug.cgi?id=1054364