oxcmsg: Implementation for partial UserDNs in ModifyRecipients call

When Outlook passes some value in RecipientRow.prefix_size, then
we should restore the full legacyExchangeDN in recipient_x500name
using szDNPrefix we supplied to Outlook during Connect op.

Signed-off-by: Kamen Mazdrashki <kmazdrashki@zentyal.com>
oxcmsg: Resolve x500dn back to sAMAccountName due to SOGo limitations atm

Despite comments, SOGo expects message_recipient->username to be
the sAMAccountName for the recipient.
Thus resolving parital x500dn to a full one brakes the backend.
Utill this is fixed, this patch is a reasonable fix for that limiatation
Merge pull request #99 from openchange/kamenim/modify-recipients-partial-x500dn-impl

RopModifyRecipients partial x500dn implementation