split_on_separator: move to a separate file
    
    To be able to include split_on_separator() without additional
    dependencies (only talloc), it is moved into a separate file.
    
    Related to https://pagure.io/SSSD/sssd/issue/3050
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>