Fix two problems in the ExecuteBinary function.

- If the process exits abnormally then we will leak the stdout and
  stderr FDs. Fix it by closing the FDs before returning.

- If another child process exits then we will incorrectly return the
  result from that process instead of waiting for our child. Fix it
  by using waitpid instead of wait.

Change-Id: I8974d5e4bd33f264cd2d364f55a60f1f5cb7eb1a