extosdep: notify external OS handler of network config updates

[aadnehovda]

This adds a small extosdep notification for network configuration changes. External OS handlers can use it as a low-cost nudge that something changed for a network and then fetch authoritative state through the existing local API.

The message carries the network ID and configuration revision. It is emitted when the service receives network configuration updates, down events, or destroy events.

This also fixes extosdep TAP IP bookkeeping in removeIp(). Without removing the deleted address from the internal allIps list, a later addIp() for the same address is incorrectly suppressed. That matters for wrappers that let ZeroTier One remain responsible for policy decisions and simply execute the resulting ADDIP/DELIP messages.

Validation

• Built ZeroTier One with ZT_EXTOSDEP=1.
• Ran it under a VyOS wrapper using zerotier-one -x fd1,fd2.
• Confirmed the wrapper received config update notifications for joined and not-found networks.
• Confirmed DELIP followed by ADDIP works when controller-managed addressing is disabled and re-enabled.

[CLAassistant]

All committers have signed the CLA.