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

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.