Add a property on NMManager indicating that it is currently starting
up and activating startup-time/boot-time network connections.

"startup" is initially TRUE, and becomes FALSE once all NMDevices
report that they have no pending activity (eg, trying to activate,
waiting for a wifi scan to complete, etc). This is tracked via a new
NMDevice:has-pending-activity property, which is maintained partially
by the device itself, and partially by other parts of the code.