vfs: simplify the `abstractvfs.rename()` implementation PyCharm was yapping about `util.rename()` not returning anything, because it is typed to return `None`, but the value was captured and returned after calling `_avoidambig()`. Instead, drop all of that, unconditionally rename, and then call `_avoidambig()` if appropriate. While we're here, convert the ersatz ternary operator into a modern one to help pytype. When a variable is initialized the old way, pytype tends to assign the type of the LHS of the `and`. In this case, that's a bool, and it will get confused that bool doesn't have a `stat` attribute once this method gets more type annotations. (Currently it thinks the `checkambig` arg is `Any`, so it doesn't care.)