oxcstor/RopLogon: Explicitly check for MAPISTATUS code
oxcstor/RopLogon: Verify EssDN for public logons

With Public Logons, EssDN is always empty (I am testing with OL 2010)
It this case, all underlying layers are failing, because there is
no valid 'username'

Proposed fix is fallback got User DN for logged in user
emsmdbp/mailbox_init: Alwasy popoulate mailbox_object object

Previously, for public folders we just missed to populate very
important fields like: EssDN and username
Merge pull request #96 from openchange/kamenim/fix-public-folders

Fix Logon for Public mailbox store