Implementation of open_memstream().

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.