Default number of jobs is unsafe (cabal-install)

Just ran `cabal new-install pandoc-include-code` without really thinking and it blew up my 16gb ram machine very quickly. Earlyoom was even unable to kill any of the processes, so I had to wait 15 minutes for the machine to become responsive again, potentially getting file corruption and break my recent work.

Number of processors is almost always the wrong choice for jobs with GHC. This is something that comes from the gcc world and even there it's sometimes wrong if the objects are too big.

The default should be **safe**, **not** *fast*. We shouldn't make random assumptions about the memory of the users machine.