Replace busybox free with *real* free from procps-ng

Busybox free reports incorrectly the used and free memory on a system. Which makes it harder to understand and compare information from other sources.

In the same effort we add some other useful procps-ng tools, like pgrep, pkill, pmap, slabtop, vmstat and watch.
Merge pull request #2211 from dagwieers/package-procps-ng

Replace busybox free with *real* free from procps-ng