Replace space: add some checks
    
    This patch adds some additional checks if the option for replacing
    spaces in user and group names is used.
    
    When replacing space with the replacement character it is checked if the
    name already contains the replacement character. If it does the
    unmodified name is returned because in this case a revers operation
    would not be possible.
    
    For the reverse operation is it checked if the input contains both a
    space and the replacement character. If this is true the unmodified name
    is returned as well, because we have to assume that it is the original
    name because otherwise it wouldn't contain both characters.
    
    Additionally a shortcut if the replacement characters is a space and
    tests for the new checks are added. The man page is updated accordingly.
    
    Related to https://fedorahosted.org/sssd/ticket/1854 and
    https://fedorahosted.org/sssd/ticket/2397 .
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 92d19f76449817dfb125da9510d478a30eed37bc)