[feature suggestion] Add ALLOW_MAKE_JOBS_{NTASKS}=yes allowing multiple jobs only when the number of tasks is low enough

[yurivict]

The problem with the current ALLOW_MAKE_JOBS=yes is that it can kill the system when too many jobs are run by poudriere (each task is run on many CPUs). The problem with ALLOW_MAKE_JOBS=nois that it is (obviously) slow when the number of tasks gets low.

I suggest to add the variable ALLOW_MAKE_JOBS_{NTASKS}=yes. For example, ALLOW_MAKE_JOBS_1=yes would run the task on multiple CPUs when it starts as a single task. This should always be safe, and can improve the overall performance.

ALLOW_MAKE_JOBS_2=yes will run on multiple CPUs when the task starts among only 1 or 2 simultaneous tasks, etc. Since a task can finish and lead to multiple other tasks, this can lead to number of jobs >NCPU. This makes such option controversial for NTASKS>=2. But IMO this option can still be useful for NTASKS>=2 as long as its effects are documented.

This option, if implemented, would make poudriere job control more flexible.

[bdrewery]

I coded this up a while ago to allow using MAKE_JOBS when only 1 job is running, or to split it for 2 jobs. It was pretty invasive for what seemed like little benefit. https://github.com/freebsd/poudriere/compare/master...bdrewery:master-make-jobs-bottlenecked?expand=1 was the implementation. By invasive I just mean it added more complexity in the queue handling which is critical code to keep simple (it's already too complex).