Protect file style [concept ok'd by mppf]

In the comment for file._style (which obtains a copy of the iostyle associated
with a specific file) it is stated that a file's style cannot be modified after
creation to prevent race conditions.  However, qio.h implemented two functions
which would break this invariant - qio_file_style_ptr, which allowed
uncontrolled access to the style; and qio_file_set_style.  To maintain the
invariant, I have removed both these methods.