SYSDB: Add sysdb_search_by_orig_dn()
    
    Three new methods have been added to sysdb's API in order to perform
    search by the orig dn (which is quite common in SSSD's code base).
    
    A common/base method called sysdb_search_by_orig_dn() is the most
    important one and then a few other helpers for searching users and
    groups groups directly.
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>