Adds support for POSIX interface open_memstream() that allows writing
to dynamic memory buffer stream. The stream is dynamically reallocated
as the buffer grows with initial size set to zero.
The caller has to free the buffer after the stream is closed.
The implementation uses fopencookie() for custom stream operations and
callbacks.
Signed-off-by: Michal Lenc <michallenc@seznam.cz>
