tailscale: set nftables as default

tailscale: set nftables as default

This changes the default firewall method used by Tailscale to nftables.
The 'autodetection' mode is only supported by arm64 and amd64 for now[1].
This causes mips devices to not do proper detection and incorrectly default back to
iptables.

I added a fw_mode variable to the tailscale.conf file that could be
set to iptables for easy conversion for someone still using iptables.
I was able to test on an older mips device and my current aarch64
without issues.

Also a few readme updates to bring it up to the current status.

1. https://github.com/tailscale/tailscale/blob/dc7aa98b768bf82017aa5cc82a62dd4d685f811d/util/linuxfw/linuxfw_unsupported.go#L4C58-L4C58

Signed-off-by: Tyler Young <git@yfh.addy.io>