Fix process007 test.

Two fixes:

    - Properly reimplement 'cat' using the 'write' system
      call instead of printf; printf was expecting a NULL-terminating
      string and was running over into the stack.

    - Use the process007_fd shim program for the second cat
      as well; essentially, apply the e380844b fix to all cases.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>