Look at NI instead of PRI in ps output on Linux

The PRI field is the Linux kernel scheduling priority which is
different from the niceness of a process (which is what a system
adminitrator and policy-writer is likely to want to select upon). This
value is almost always larger than 20, so it isn't selectable if the
defined range per the documentation is -20,+20 anyway.
Merge pull request #400 from jeramey/master

Look at NI instead of PRI in ps output on Linux