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.
Enhance the child_error() function so that it will write error output to the
child's sync output buffer, if it exists. If it doesn't the output goes to
stdout/stderr.
A new flag to the -O/--output-sync, "job", selects a per-job (that is, per
line of a recipe) output synchronization. To support this move the close of
the temp file out of the sync_output() function and don't do it until we free
the child, since we may call sync_output() multiple times in a given recipe.
When we set up for a new temp file, if we're in per-job mode we truncate the
file and seek to the beginning to re-use it for every job.
