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).

testing/btest/Baseline/scripts.base.utils.exec/bro..stdout

@@ -3,5 +3,4 @@ test1, [exit_code=0, signal_exit=F, stdout=[done, exit, stop], stderr=<uninitial
 [out2] = [insert more text here, and there]
 }]
 test2, [exit_code=1, signal_exit=F, stdout=[here's something on stdout, some more stdout, last stdout], stderr=[and some stderr, more stderr, last stderr], files=<uninitialized>]
-test3, [exit_code=9, signal_exit=F, stdout=[FML], stderr=<uninitialized>, files=<uninitialized>]
 test4, [exit_code=0, signal_exit=F, stdout=[hibye], stderr=<uninitialized>, files=<uninitialized>]