vis: adding a pure golang Vis()

The current Vis() and Unvis() are using the C implementation from
MTREE(8).

But that means that cgo is used, which is not always desired.

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

walk.go

@@ -162,7 +162,7 @@ func Walk(root string, excludes []ExcludeFunc, keywords []Keyword) (*DirectoryHi
 				}
 			}
 		}
-		encodedEntryName, err := Vis(entryPathName)
+		encodedEntryName, err := Vis(entryPathName, DefaultVisFlags)
 		if err != nil {
 			return err
 		}