Changing NM_CONTROLED from "no" to "yes" worked just the first time.
Fix that by storing unmanaged spec when interface becomes unmanaged
and adjust condition identifying "no-change" updates to the ifcfg
file.
When a connection changes on-disk, the in-memory copy of it may contain
transient secrets (agent-owned or not saved) that dont' get written out
to disk. When comparing the on-disk copy to the in-memory copy make sure
transient secrets are ignored so that we don't re-read the on-disk copy
needlessly.
The signal was emitted in case the removed connection was managed instead of
for unmanaged connection. Thus the signal had no effect.
That caused incorrect behaviour in case of changing NM_CONTROLLED=no to yes.
That didn't enable the device; only after the file was changed for the second time.
