pager: rework D/discard buffer

The previous solution had the issue that it switched between "delete
buffer, then move back" and "delete buffer, then move forward" depending
on whether the buffer was the root of the buffer tree, which made its
behavior quite unpredictable.

Now the pager (sort of) remembers the direction you are coming from,
and D moves in that direction. So e.g.:

* Enter, D just moves back to where you were coming from (as before)
* Comma, D deletes the previous buffer, then returns to the current
  buffer

If no buffer exists in the target direction, then we alert.

Also, new commands are: `d,' `d.'. They do the same thing the
non-d-prefixed variations do, but also delete the current buffer. Useful
if you're no longer sure where you are coming from, but know where you
want to go. (`d,' in particular is equivalent to w3m's `B'.)

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