proctrack: conversion of container ID to 64 bit

This continues the conversion of the cont_id from 32bit to 64bit,
 * updated slurm_container_find() to return u64 in order to match type of container ID;
 * updated slurm_proctrack_ops() to match the update of u32 -> u64 container ID in the
   slurm_container_xxx() functions,
 * miscellaneous type conversions from/to u64,
 * using "%"PRIu64"" for printing 64 bit.
Patch 01_proctrack-64-bit-conversion.diff from Gerrit Renker