DDMR fix for indexed mode reading (not newscan): Showing the first unread message when selecting a sub-board, updating scan pointer, and selecting the next sub-board when done with the current sub-board

DDMR fix for indexed mode reading (not newscan): Showing the first unread message when selecting a sub-board, updating scan pointer, and selecting the next sub-board when done with the current sub-board.

For indexed read mode (not doing a newscan), when choosing a sub-board to read, the correct (first unread) message is displayed. Also, the user's scan pointer is also updated to the last_read pointer. When done with the current sub-board, the next sub-board in the indexed menu is selected.