Behavior of symlink on BasePathFs

[rony36]

Symlinks on BasePathFs behave not match my expation. SymlinkIfPossible will resolve the RealPath of underlayer filesystem before make a symlink. For example, craeting a link like "/foo/bar" -> /foo/file" will be transform into a link point to "/{basepath}/foo/file. This makes following issues.

1. SymlinkIfPossible is not match to ReadlinkIfPossible Read a linke create by SymlinkIfPossible, will return "/{basepath}/foo/file", but not "/foo/file"

2. Related path It should be ok for related path, like "link" -> "../file"