The original compel_prepare() also initializes the infect_ctx with
values suitable for simple usage. As a starting point the task_size
value is set.

The compel_prepare_noctx() allocates ctx-less handler that is to be
filled by the caller (CRIU).

travis-ci: success for compel: Contrinue improving library
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>