security/tailscale: Robustify rc.d script

Check for orphaned network interface at the time of start, and if such
an interface exists then destroy it before starting tailscaled. When
tailscaled terminates unexpectedly it fails to cleanup, leaving the
orphaned interface behind. This results in it failing to start until
the interface is cleaned up, making it unsuitable for being monitored
by a service monitoring software (e.g. monit)

Reported by: ler