Until we handle bridges non-destructively, only manage bridges
created by NM.  When quitting write out a file listing all
bridges created by NM and a timestamp, and when starting read
that file and if the timestamp is within 30 minutes, manage
any bridge that was listed in that file.  This scheme, while
not foolproof (eg, if NM crashes), should ensure that NM can
recognize bridges it created if it's restarted.  The file
is stored in /run or /var/run, which is cleaned each restart,
ensuring that the state does not persist across reboots.

If an automatic or user-initiated activation request for
a bridge NM does not manage is received, that request is
denied.  Only if the bridge interface does not yet exist, or
was present in the managed bridges file, will an
NMDeviceBridge be created and activation be possible.