firewall: refactor "nm-firewall-manager.c" to not use GDBusProxy
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Don't use GDBusProxy but plain GDBusConnection. NMFirewallManager is very simple, it doesn't use any of the features that GDBusProxy provides.
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make NMFirewallManagerCallId typedef a pointer to the opaque call-id struct, instead of the struct itself. It's confusing to have a variable that does not look like a pointer and assigning %NULL to it.
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internally drop the CBInfo typename and name the call-id variable constsistantly as "call_id".
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no need to keep the call-id struct alive after cancelling it. That simplifies the lifetime managment of the pending call because the completion callback is always invoked shortly before destroying the call-id.
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note that the caller is no longer allowed to cancel a call-id from inside the completion callback. That just complicates the implementation and is not necessary. Assert against that.