Add support for per-job output sync.

Add support for per-job output sync.

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.