oxcmsg: Implement using 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>