textiter: make the FindLogAttrFunc functions clearer

- Return true (found) and false (not found) explicitly.
- Set found_offset only when something has been found.

find_backward_cursor_pos_func() was a bit different, the while loop had
the condition "offset > 0" but the return was "offset >= 0". Probably a
micro-optimization, since offset == 0 is always a cursor position.
Anyway now the code is the same as the other functions.