Fix rump.ping6 -c N (N > 1) doesn't work

2nd packet and subsequent packets are sent based on SIGALRM and
it depends on poll(2) returns with EINTR by the signal. However,
currently poll is rump-ified while signals aren't so the signal
doesn't wake up poll and ping6 doesn't work expectedly.

Rump-ifying signals is unsure (nobody does it for now) and the
combination use of signals and poll makes the logic a bit complex.
So let's fix the defect by stopping using signals for packet
transmissions. The new logic is derived from ping(8).

Bonus: ping6 -i 0.01 works as we expect now while the original
didn't work enough fast.

(ozaki-r)