Use a larger amount of data for tests such as the interrupted_write

tests that depend on filling up an OS pipe so that they work properly
on systems configured with large pipe buffers.

Also a subprocess docstring update that i forgot was in my client when
i did the original 3.3 commit... easier to just leave that in here
with this one than go back and undo/redo.

diff --cc Lib/subprocess.py
Simple merge

diff --cc Lib/test/support.py
Simple merge