lib: [procutils.c]: general purpose procfs parsing functions

lib: [procutils.c]: general purpose procfs parsing functions

We include the following functions as a first approach:

  - proc_open_tasks(): allocate resources and setup basic data
  - proc_next_tid(): iterate over the thread group
  - proc_close_tasks(): free used resources

[kzak@redhat.com: - free() on error in proc_open_tasks(),
                  - always call free() in proc_close_tasks()
                  - ignore non-digit dirs and return 1 on end-of-dir
                    in proc_next_tid()]

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Karel Zak <kzak@redhat.com>