wgengine: don't allocate so much in userspaceEngine.getStatus

It was one of the top garbage producers on my phone.

It's slated to be deleted and replaced anyway, but this helps in the
meantime.

The go.sum changes look scary, but the new dep only adds 240 bytes to
the binary. The go.sum noise is just cmd/go being aggressive in
including a lot of stuff (which is being fixed in Go 1.15, for what I
understand). And I ran a go mod tidy, which added some too. (I had to
write a custom wrapper around go mod tidy because this mod tidy
normally breaks on tailscale.io/control being missing but referenced
in tests)

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

go.mod

@@ -16,12 +16,11 @@ require (
 	github.com/kr/pty v1.1.1
 	github.com/mdlayher/netlink v1.1.0
 	github.com/pborman/getopt v0.0.0-20190409184431-ee0cd42419d3
-	github.com/peterbourgon/ff v1.7.0 // indirect
 	github.com/peterbourgon/ff/v2 v2.0.0
-	github.com/tailscale/hujson v0.0.0-20190930033718-5098e564d9b3 // indirect
 	github.com/tailscale/winipcfg-go v0.0.0-20200213045944-185b07f8233f
 	github.com/tailscale/wireguard-go v0.0.0-20200407164751-7f0c43dd1145
 	github.com/toqueteos/webbrowser v1.2.0
+	go4.org v0.0.0-20200406031205-0882d5f8a577
 	golang.org/x/crypto v0.0.0-20200317142112-1b76d66859c6
 	golang.org/x/net v0.0.0-20200301022130-244492dfa37a // indirect
 	golang.org/x/oauth2 v0.0.0-20200107190931-bf48bf16ab8d
@@ -29,6 +28,5 @@ require (
 	golang.org/x/sys v0.0.0-20200317113312-5766fd39f98d
 	golang.org/x/time v0.0.0-20191024005414-555d28b269f0
 	gortc.io/stun v1.22.1
-	honnef.co/go/tools v0.0.1-2020.1.3 // indirect
 	rsc.io/goversion v1.2.0
 )