(core+) minor fixes from buffer-stream experiments

Added fallback notification route if underlying graphics layer
did not accept the buffer we received from a client (incompatible
buffers or possible indication of exploitation attempt) that
disables the feature for the specific frameserver segment.

More cleanup- hooks to make sure that egl-dri and linux input
event platforms recover after some terminal state transitions
(notably tired of sshing in, fixing things then rebooting).

re-ordered frameserver structure to be a bit more careful,
extended type model for segments to cover VM, HMD_L/R and
rename SHELL to TERMINAL because my terminology is apparently
not in synch with the rest of the world.

Fixed streaming- source regression related to non-3d libretro
sessions, preliminary buffer passing for 3d libretro but something
is broken in the chain [libretro-fsrv] [libretro-spec] [core-behavior]
(one suspect is frame-duplication).

Moved SYSTEMPATH libretro- bits to be part of the ARCAN_ARG
structure rather than environment expected to be inherited
from parent (should really cut down on that code pattern).