Use /proc/<pid>/smaps to calculate mem on linux to allow usage of PSS.

[ksoderstrom]

This PR changes the behavior of the Linux memory calculation to use /proc/<pid>/smaps instead of /proc/<pid>/status. This allows the user to request either RSS or PSS memory. PSS is the process' proportional share of the mapping that is resident in RAM and is calculated by the Linux kernel.

The default value is still rss, and pss is only compatible with Linux.

[schneems]

❤️ thanks again for this! One thing i noticed is that ps command doesn't have a pss report. I'm thinking that we should default the metric to report to pss on linux, and return rss always on a mac as there's no equivalent pss (that I can find).

[ksoderstrom]

Right, I couldn't find any alternatives for OS X. I suppose it would be possible to calculate it, since it's more or less based on shared pages / num processes sharing it. Not sure if it's possible to get that from ps though.

Changing the default for linux might make sense. I felt it would be a bit ugly to get totally different results with default options on different platforms, but I can create a PR if you want to.