And yay: tsnet (and thus k8s-operator etc) no longer depends on
portlist! And LocalBackend is smaller.
Removes 50 KB from the minimal binary.
Updates #12614
Change-Id: Iee04057053dc39305303e8bd1d9599db8368d926
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>