vis/unvis: pull in exact implementation from FreeBSD

Perhaps this is not completely ideal, because it brings in cgo.
And with the flags, it can have tailored experience.

I've added a basic test to ensure that the cases we're interested in are
covered.

This does not yet integrate the usage of Vis()/Unviz() into the manifest
create and compare.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

vis_test.go

@@ -0,0 +1,35 @@
+package mtree
+
+import "testing"
+
+func TestVis(t *testing.T) {
+	testset := []struct {
+		Src, Dest string
+	}{
+		{"[", "\\133"},
+		{" ", "\\040"},
+		{"	", "\\011"},
+	}
+
+	for i := range testset {
+		got, err := Vis(testset[i].Src)
+		if err != nil {
+			t.Errorf("working with %q: %s", testset[i].Src, err)
+			continue
+		}
+		if got != testset[i].Dest {
+			t.Errorf("expected %#v; got %#v", testset[i].Dest, got)
+			continue
+		}
+
+		got, err = Unvis(got)
+		if err != nil {
+			t.Errorf("working with %q: %s", testset[i].Src, err)
+			continue
+		}
+		if got != testset[i].Src {
+			t.Errorf("expected %#v; got %#v", testset[i].Src, got)
+			continue
+		}
+	}
+}