cg_build_path adds needless "/" character

cg_build_path adds needless "/" character

    cg_build_path adds needless "/" character in cases when the input path
    ends/starts with character "/" itself. This character is added now only
in cases it is wanted and it was not set.

    Changelog
    - incorporate Jan's feedback - fix typo in patch description, use snprintf instead of sprintf
    - deal with the situation if directory name is empty as well
    - move the path concatenation functionality to a separate function

    Example:
    Old version:
    cg_build_path("", path, memory);
    returns /sys/fs/cgroup/memory//
    cg_build_path("/3", path, memory);
    returns /sys/fs/cgroup/memory//3/
    cg_build_path("3/", path, memory);
    returns /sys/fs/cgroup/memory/3//

    New version:
    cg_build_path("", path, memory);
    returns /sys/fs/cgroup/memory/
    cg_build_path("/3", path, memory);
    returns /sys/fs/cgroup/memory/3/
    cg_build_path("3/", path, memory);
    returns /sys/fs/cgroup/memory/3/

    Signed-off-by: Ivana Hutarova Varekova <varekova@redhat.com>
    Acked-by: Dhaval Giani <dhaval.giani@gmail.com>