lib/procfs: add functions to read /proc/#/ stuff

lib/procfs: add functions to read /proc/#/ stuff

This is extension to lib/path.c (the same as lib/sysfs.c for block
devices) to read info about processes.

* replaces code from lib/procutils.c
* remove proc_{open,close}_processes(), opendir and readdir is good enough
* add procfs_dirent_*() functions to work with readdir() from /proc
* add new path_cxt based API (will be extended in next commits) for
  complex tasks

Signed-off-by: Karel Zak <kzak@redhat.com>