controlclient/netmap: write our own b.ConciseDiffFrom(a) function.

This removes the need for go-cmp, which is extremely bloaty so we had
to leave it out of iOS. As a result, we had also left it out of macOS,
and so we didn't print netmap diffs at all on darwin-based platforms.
Oops.

As a bonus, the output format of the new function is way better.

Minor oddity: because I used the dumbest possible diff algorithm, the
sort order is a bit dumb. We print all "removed" lines and then print
all "added" lines, rather than doing the usual diff-like thing of
interspersing them. This probably doesn't matter (maybe it's an
improvement).

ipn/ipnserver/server.go

@@ -18,7 +18,6 @@ import (
 	"syscall"
 	"time"
 
-	"github.com/google/go-cmp/cmp"
 	"github.com/klauspost/compress/zstd"
 	"tailscale.com/control/controlclient"
 	"tailscale.com/ipn"
@@ -112,7 +111,6 @@ func Run(rctx context.Context, logf logger.Logf, logid string, opts Options, e w
 	b.SetDecompressor(func() (controlclient.Decompressor, error) {
 		return zstd.NewReader(nil)
 	})
-	b.SetCmpDiff(func(x, y interface{}) string { return cmp.Diff(x, y) })
 
 	var s net.Conn
 	serverToClient := func(b []byte) {