zdtm: add autofs test

1) Support autofs kernel-user exchange protocol
2) Test mounts: direct, indirect, offset (?) (migrated and operational).
3) Test cases, when write end of the pipe is closed, and when it's not.
4) Test catatonic mounts.
5) Test nested mounts are migrated (tmpfs suits it good)

Signed-off-by: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>