ping: fix write-after-allocated-mem bug

ping: fix write-after-allocated-mem bug
ping: use monotonic_us instead of gettimeofday: smaller code and
      needs only 4 bytes in the packet
ping: display roundtrip times with 1/1000th of ms, not 1/10 ms precision.
wget: small optimization

function                                             old     new   delta
pingstats                                            243     259     +16
sendping6                                             98      93      -5
sendping4                                            183     178      -5
.rodata                                           129715  129707      -8
progressmeter                                        867     855     -12
unpack_tail                                          320     272     -48
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(add/remove: 0/0 grow/shrink: 1/5 up/down: 16/-78)            Total: -62 bytes