Warn if `--jobs` is too high

[sourcefrog]

It's easy for people to assume they should use -j NPROC but that's almost always a mistake, because e.g. 32 parallel cargo jobs will probably melt your machine, because each cargo process itself starts many threads and jobs.

The recent release of jobserver support should help mitigate this, and the manual does warn against it, but it's such a easy mistake to make and a bad experience so perhaps it should be blocked.

1. Warn if -j is more than say 4
2. Or maybe just refuse to launch more than say 4 jobs unless also given --many-jobs