checkpoint: preserve in-memory state of connections

If a connection is in-memory (i.e. has flag "unsaved"), after a
checkpoint and rollback it can be wrongly persisted to disk:

 - if the connection was modified and written to disk after the
   rollback, during the rollback we update it again with persist mode
   "keep", which keeps it on disk;

 - if the connection was deleted after the rollback, during the
   rollback we add it again with persist mode "to-disk".

Instead, remember whether the connection had the "unsaved" flag set
and try to restore the previous state.

However, this is not straightforward as there are 4 different possible
states for the settings connection: persistent; in-memory only;
in-memory shadowing a persistent file; in-memory shadowing a detached
persistent file (i.e. the deletion of the connection doesn't delete
the persistent file). Handle all those cases.

Fixes: 3e09aed2a0 ('checkpoint: add create, rollback and destroy D-Bus API')

NEWS

@@ -15,6 +15,8 @@ USE AT YOUR OWN RISK.  NOT RECOMMENDED FOR PRODUCTION USE!
   visible in nmcli via "nmcli -f all device show $DEV".
 * Deprecated 802-11-wireless and 802-11-wired property 'mac-address-blacklist'
   and introduced the 'mac-address-denylist' property.
+* Properly restore in-memory connection profiles during the rollback
+  of a checkpoint.
 * Fix detection of 6 GHz band capability for WiFi devices
 * Allow IPv6 SLAAC and static IPv6 DNS server assignment for modem broadband
   when IPv6 device address was not explicitly passed on by ModemManager