network: drop sections contain invalid settings in network_verify()

network: drop sections contain invalid settings in network_verify()

If e.g., an [Address] section has an invalid setting, then
previously assigned settings in the section is freed, and
only later settings are stored. That may cause partially broken
section stored in Network object.

This makes if an invalid setting is found, then set 'invalid' flag
instead of freeing it. And invalid sections are dropped later by
network_verify().