Improve sync handling for -Ojob/-Otarget and recursion.

Improve sync handling for -Ojob/-Otarget and recursion.

If we are not going to sync a command line then dump any collected output
first to preserve ordering.  Do some code cleanup:
  * Move the handle init to a separate function.
  * Move the temp file truncation to the output function.
  * Remember whether we sync in a variable for readability.
  * Handle EINTR and short writes in child_out().
  * Always call sync_output() in case output_sync was changed due to error.