Support procps 4 API

procps 4.0.0 has just been released, and it no longer provides the library with the readproc API that igt-gpu-tools uses. The release announcement says it's "probably not ready for the main path of most distributions", but I guess it'll start showing up in the more adventurous distributions reasonably soon...

There's a new library libproc-2, with a different API for fetching process information: see pids.h for the interface, and (say) pgrep.c for an example of use. It looks like it should be reasonably straightforward to port the igt-aux.c code to use it, but it'll probably need conditionals to support both libraries.