stream: allocate enough space for select() hack

`fd_set`s are way too small for `select()` hack when stream's fd is
bigger than 1023. Make `fd_set`s a part of `uv__stream_select_t`
structure.

Should be backported to all branches up to (and including) v0.10
Mode the comment above this line to the right place, that is, before creating the socketpair.