New field 'l' in qw_fb_t for the end-of-line character.

So now special character processing is done in qw_fb (where it
belongs to) instead of in the drivers.
Set default end-of-line char to standard paragraph separator.
New function op_undo().

But undo is crippled for 2 reasons:

 - e->ac is not saved, so after undo the cursor is in an undefined
   position;
 - There is no way of detecting boundaries (no more to undo).
Also recover the ac position (from j->o) when undoing.
Do not create an empty block in qw_edit_new().
Allow NULL blocks in qw_blk_to_abs().
Added a special mark (NOP operation) as the first journal entry.

This is to avoid overpassing the "no-more-to-undo" boundary. It
has been marked as an ugly hack.