file_read_string changes

Using a parent pointer is conventional but if desired, we could keep the existing single-param interface to file_read_string and nih_ref() from the caller to give the same effect.

Also, not sure about your view on the call to drop_newlines()? It seems reasonable for single-line files as cgmanager_get_value() then returns just the value but maybe not reasonable for multi-line files to drop the final NL?