all screen implementations use a Memimage* internally
for the framebuffer, so we can return a shared reference
to its Memdata structure in attachscreen() instead of
a framebuffer data pointer.
this eleminates the softscreen == 0xa110c hack as we
always use shared Memdata* now.
making sure to close the dot in every kproc appears repetitive,
so instead stop inheriting the dot in kproc() as this is usually
never what you wanted in the first place.
