options on the command line now define *defaults* that host files can
override (for instance -j, stuck, -p, -J).
Add -p /n to mean "take number of jobs, if >1, divide by n, round up to 2,
and use that for parallel.
Document -p.
Make junk be 'by host' (and it's a prop, so you can tweak it).
concurrent log that records how many jobs are running each time it changes.
tag parallel builds *n in the time record.
cdc017bcb0