[review] dcbw/mbm-connect-disconnect-cleanup: simplify and clean up mbm plugin connect and disconnect

[Dan Williams]

Hi,

Please review the dcbw/mbm-connect-disconnect-cleanup branch.

https://cgit.freedesktop.org/ModemManager/ModemManager/log/?h=dcbw/mbm-connect-disconnect-cleanup

Git commit comments should have a pretty good overview of what's going
on; these were spawned out of the following bugs that Lubomir found
doing some automated testing:

https://bugs.freedesktop.org/show_bug.cgi?id=95302
https://bugs.freedesktop.org/show_bug.cgi?id=95304

Dan