Libxo bugs and fixes.

[Phil Shafer]

Marcel Moolenaar writes:
>What's wrong with just calling fflush()?

My take is that the missing piece was the ability of xo_flush to
flush the underlaying (opaque) stream.  The way it's currently coded
(the "develop" branch) adds the flush function to xo_set_writer and
uses fflush for stdout, but doesn't call __flbf.  The app needs to
decide when a flush is needed and calls xo_flush.

Setting the XOF_FLUSH flag should (but doesn't yet) trigger this
for each xo_* call.  Currently XOF_FLUSH just flushes to the
writer, but doesn't call the flush function.  I'll add this.

Thanks,
 Phil