New API: boro_node_r(), boro_node_u(), boro_node_d() and boro_node_f().
Changed all code to the new API.
Made boro_node_d() to return NULL.
More API changes.
Deleted boro_node_f() and replaced it by calls to b->flush().

This operation should not be public, so it's better to leave it as
something special.
Added new driver function b->headr() to be called whenever the header is changed.
Fixed all calls to b->headr() (brrrrr...)
Made b->disp() mandatory.
Added some coding ideas for the new mmaped driver.