Only implements the POSIX jobserver and will break on windows.
Unbreaking it on windows will be done as a follow up.
Not used for autotune, that needs some more changes.
* Rename and move sbysrc/demo[123].sby to docs/examples/demos
* Make them use multiple tasks for multiple engines
* Scan docs/examples for sby files for make test
* `make ci` is now `NOSKIP` by default
* Skip scripts using `verific` w/o yosys verific support
* This does not fail even with NOSKIP set
