bluetooth: signal devd on netgraph device events

[charlesrocket]

Currently, devd emits events for external adapters only. Send Netgraph init/disconnect events to devd so the internal adapter's state could be asserted from userland.

[github-actions]

Thank you for taking the time to contribute to FreeBSD!

All issues resolved.

[charlesrocket]

Motivation

I needed a piece of code that could listen for bluetooth adapter's state changes in userland (event-driven exclusively, absolutely no polling). Since this is a built-in/static adapter (T480), devd does not produce any events on "power cycle" (one needs to physically pull the device out/disconnect to trigger these events, which is not possible with soldered devices). The only other way to catch these changes is via netgraph, but it requires root.

With this patch, netgraph sends "power cycle" events (NGM_HCI_NODE_INIT case and ng_hci_disconnect() call) to devd, allowing userland to access these events (kqueue on the socket/directly/etc).