Allow mass delete for hidden nonames only

The delete all hidden buffers function has changed to delete only hidden
no name buffers. Hidden buffers are essential from the plugin point of
view so the user is encouraged to work with them closely. Therefore
mass delete of them has been disabled. Instead, the D binding now
enables to mass wipeout of hidden and not modified no name buffers.

It is safe to treat F and D as secure clean up commands.