Add an implementation of open_memstream

[dddejan]

It can be implemented using funopen on BSD

[ianamason]

Yes that how it is done here:

http://piumarta.com/software/memstream/

[dddejan]

This should be fixed in open_memstream branch. Some testing required.

[dddejan]

to look at: https://github.com/Snaipe/fmem