We currently use more complex and functional protocol on linux,
and a simple ad-hoc protocol on other OSes.
This leads to code duplication in both ipc and executor.
Linux supports coverage, shared memory communication and fork server,
which would also be useful for most other OSes.
Unify communication protocol and parametrize it by
(1) use of shmem or only pipes, (2) use of fork server.
This reduces duplication in ipc and executor and will
allow to support the useful features for other OSes easily.
Finally, this fixes akaros support as it currently uses
syz-stress running on host (linux) and executor running on akaros.
This breaks circular dependency between:
sysgen -> sys/linux -> sys -> sysgen
With this circular dependency it is very difficult to
update format of generated descriptions because sysgen does not build.
