restorer: Report child's death reason correctly

E.g, if child was killed by SIGSEGV, this message
previously was "exited, status=11", as si_code == CLD_DUMPED == 3
in this case will result in (si_code & CLD_KILLED) == (si_code & 1).
Which is misleading as you may try to look for exit() calls
with 11 arg.
Correct if to compare si_code with CLD_*.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>