RFR: 8229872: (fs) Increase buffer size used with getmntent

[Florian Weimer]

* Alan Bateman:

> Can you move rewind to UnixNativeDispatcher so that it's with the
> other FILE* functions? I'm pretty sure you have to set errno to 0
> before calling it and it will set it if there is an error.

You can check for errors using ferror.

> I think UnixNativeDispatcher should also define getline to wrap the
> libc function rather that getlinelen.

That requires defining a new struct, for the internal state (buffer
pointer and length).

Thanks,
Florian