Assign name to mailbox

        Entry:
          AH = 12h
          BH = 11h
          BL = mailbox to name
               00h DWORD on top of stack is mailbox handle
               02h use given task's mailbox (task's handle on top of stack)
               03h use current task's default mailbox
          STACK: DWORD length of name
                 DWORD address of name