put sid_to_name behind the winbindd backend interface

put sid_to_name behind the winbindd backend interface
I spent quite a while trying to work out how to make this call
via ldap and failed. I then found that MS servers seem use rpc
for sid_to_name, and it works even when in native mode, I ended
up just implementing it via rpc
(This used to be commit 789833b44e342c0b5de463ed8f9b5f7474a99f27)