Exec module and raw input reader fixes.

- Do stream mode for commands done by exec module, it seems important
  in some cases (e.g. ensure requested stdin is fully written).

- For cases where the raw input reader knows the child process has been
  reaped, set the childpid member to a sentinel value to indicate such
  so we don't later think we should kill it or wait on it anymore.

- More error checking on dup2/close calls.  Set sentinel values when
  closing ends of pipes to prevent double closing a fd.

- Signal flag not set when raw input reader's child exits as a result
  of a signal.  Left out a test for this -- might be portability issues
  (e.g. Ubuntu seems to do things different regarding the exit code and
  also is printing "Killed" to stderr where other platforms don't).
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Exec module and raw input reader fixes.