Use SymbolicLink rather than Symlink in API

Rename followInsecureSymlinks to followInsecureSymbolicLinks. This
now matches what is writhe in the readme and release notes.

https://github.com/kriskowal/q-io/blob/master/README.md#filetreepath-options--application
https://github.com/kriskowal/q-io/blob/master/CHANGES#L55