basic/cgroup-util: introduce cg_get_keyed_attribute_full()
    
    Callers of cg_get_keyed_attribute_full() can now specify via the flag whether the
    missing keyes in cgroup attribute file are OK or not. Also the wrappers for both
    strict and graceful version are provided.
    
    (cherry picked from commit 25a1f04c682260bb9b96e25bdf33665d6172db98)
    
    Related: #1830861
    
    patch_name: 0419-basic-cgroup-util-introduce-cg_get_keyed_attribute_f.patch
    present_in_specfile: true
    location_in_specfile: 419
    squash_commits: true