policy: add "most shared path" policies

Like path preserving but walks back the path till a match is found. Should
cover the usecase where someone wants a "less strict" form of path
preservation.

src/fs_path.cpp

@@ -29,7 +29,7 @@ namespace fs
 {
   namespace path
   {
-     string
+    string
     dirname(const char *path_)
     {
       string path(path_);