Implement support for the GNU Make jobserver

[cryptk]

It's pretty common to use a task file to invoke building other dependencies, and those other dependencies frequently use systems like make or cmake. It would be nice if task file was able to run the same style job server that gnu make runs so that parallelism could be controlled across all of these invocations.

This would allow running a task with a concurrency value of something like 20, and you would be able to ensure that even if some of those tasks call out to GNUmake (or other job server compliant parallel tooling), you still won't exceed 20 threads.

Here's a page detailing how the job server protocol works https://www.gnu.org/software/make/manual/html_node/Job-Slots.html

[trulede]

@cryptk I would suggest to implement a small CLI app to implement a block/fence mechanism in Go lang. Then you can achieve the effect:

tasks:
foo:
cmds:
- request-job 10 # request 10 CPU, block until available
- defer: cmd: release-job
- build commands

When its working, see about how to integrate that with Task; i.e. a Command Design Pattern or direct code support.