Add total system memory size to INFO

This was https://github.com/antirez/redis/pull/1820, but I broke GitHub by renaming the branch and I can't re-open the old one.

Another feature we should include with this PR: create `used_memory_rss_human` field in INFO since it's *really* easy to overlook that metric in `info memory`.  Every other memory field has a `_human` output (except lua memory too)

This PR will help us resolve issues like https://github.com/antirez/redis/issues/2217 much easier without having to ask everybody "how much memory total?  which eviction policy?"