crond: reap orphaned grandchildren to prevent zombie buildup

If a cron job launches a background task, e.g. `sh -c "sleep 5 &"`,
the shell exits immediately and the `sleep` process is re-parented to
PID 1.  When BusyBox `crond` itself happens to be PID 1 (common in a
minimal container), those orphans become direct children of `crond`.
Because `crond` only calls waitpid() for the PIDs it explicitly tracks,
these processes remain forever in Z state and the container slowly
fills with zombies.

Add a small `while (waitpid(-1, NULL, WNOHANG) > 0)` sweep at the end
of check_completions() so any stray children are reaped.  When `crond`
is not PID 1 the loop returns -ECHILD immediately, so behaviour and
overhead on a normal system are unchanged.

Size impact: +12 bytes on x86-64 (gcc 13.3.0 -Os, static)

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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