We need a distinction between external activations and assuming
connections. The former shall have the meaning of devices that are
*not* managed by NetworkManager, the latter are configurations that
are gracefully taken over after restart (but fully managed).

Express that in the activation-type of the active connection.

Also, no longer use the settings NM_SETTINGS_CONNECTION_FLAGS_VOLATILE
flag to determine whether an assumed connection is "external". These
concepts are entirely orthogonal (although in pratice, external
activations are in-memory and flagged as volatile, but the inverse
is not necessarily true).

Also change match_connection_filter() to consider all connections.
Later, we only call nm_utils_match_connection() for the connection
we want to assume -- which will be a regular settings connection,
not a generated one.