crtools: Cleanup threads stuff parsing

Move the whole code dealing with parsing threads at collect_pstree
stage into one function.

The nr_threads calculation is performed inside it based on the amount
of directories in /proc/pid/task, not the Threads: count in status.

Error from xrealloc is ignored by now.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>