POSIX seems to have chosen open_memstream() over the BSD variant. We
want something for Dalvik that will work on both GNU/Linux and Android,
so this is open_memstream() implemented in terms of BSD funopen().
For Windows there's just a stub that calls abort().
I'm putting this in libcutils since it seems inappropriate for bionic
(which provides the BSD alternatives) but isn't Dalvik-specific.
