What does this implement/fix?

The existing implementation for /api/network/{gateway,interface} relied on manually parsing various files in /proc/ and /sys/class/net. This had several drawbacks, e.g., was it not possible to get IPv6 routes (and, hence, gateways) or lifetimes and priority of IPv6 addresses.

This PR instead implements Netlink which is the proper way of querying the kernel in a standardized way about routes and interface details. It is also the same method used by the popular ip command (e.g. ip a and ip r):

Netlink is designed and used for transferring miscellaneous networking information between the kernel space and userspace processes. Networking utilities, such as the iproute2 family and the utilities used for configuring mac80211-based wireless drivers, use Netlink to communicate with the Linux kernel from userspace. Netlink provides a standard socket-based interface for userspace processes, and a kernel-side API for internal use by kernel modules. Originally, Netlink used the AF_NETLINK socket family.

Netlink is designed to be a more flexible successor to ioctl; RFC 3549 describes the protocol in detail.

This change allows us to provide much better/more complete interface information through the API offering the base for implementing this Discourse feature request.

Note: The prefered (one r) typo is a standard typo in Linux and expected.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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