Tries to simplify code, and buffer is not emptied unless we really read
more data (useful for edge reads). get_offset is now reliable (though
never used).
It was implemented for Audacious only to open companion NWA files, but
current version has no need for it. In case there is some obscure thing
that needs it, Audacious's vfs.cc should translate or return realnames
transparently, without having to modify other streamfiles.
