core: fix master deactivation
NMActiveConnection was categorizing all deactivation of master connections as "failure", and NMActRequest was deactivating all of the master's slaves with REASON_DEPENDENCY_FAILED no matter what the real reason was. In fact, NMActiveConnection only needs to handle the cases where the master fails before enslaving the device; any failure after that point will be caught by existing master/slave checks in NMDevice. So update the code accordingly (and remove the master_failed code from NMVpnConnection entirely, since no master supports having VPN slaves).
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