Refactor: libcrmcommon: add procfs handling functions

This adds a new file, procfs.c, with functions for handling procfs file systems
(/proc), and a corresponding (non-public) procfs.h included by crm_internal.h.
It includes two functions currently (based on existing procfs handling),
for reading process name and ID information from procfs.

This will help reduce duplicated code and provides a place to centralize some
OS-specific handling.