Since commit 2d1b85f8 (th/assume-vs-unmanaged-bgo746440), we clearly
distinguish between two modes when encountering devices with external
IP configuration:
 a) external devices. For those devices we generate a volatile in-memory
   connection and pretend it's active. However, the device must not be
   touched by NetworkManager in any way.
 b) assume, seamless take over. Mostly for restart of NetworkManager,
   we activate a connection gracefully without going through an down-up
   cycle. After the device reaches activated state, the device is
   considered fully managed. For this only an existing, non volatile
   connection can be used.

Before 'th/assume-vs-unmanaged-bgo746440', the behaviors were not
clearly separated.

Since then, we only choose to assume a connection (b) when the state
file indicates a matching connection. Now, extend this to also assume
connections when:

  - during first-start (not after a restart) when there is no
    state file yet.
  - and, if we have an existing, non volatile, connection which
    matches the device's configuration.

This patch lets NetworkManager assume connection also on first start.
That is for example useful when handing over network configuration from
initrd.

This only applies to existing, permanent, matching(!) connections, so it is a
good guess that the user wants NM to take over this interface. This brings us
closer to the previous behavior before 'th/assume-vs-unmanaged-bgo746440'.

https://bugzilla.redhat.com/show_bug.cgi?id=1439220