MFC r271910, r273211:

• Add -x waittime and -X timeout options for feature parity. These are equivalent to -W and -t options of ping(8). Different letters are used because both have already been used for another purposes in ping6(8).

• Fix a problem that reply packets are not received when -i T option is set and (T < RTT).
  • Use select(2) for timeout instead of interval timer. Remove poll(2) support.
  • Use sigaction(2) instead of signal(3).
  • Exit in SIGINT handler when two signals are received and doing reverse DNS lookup as ping(8) does.
  • Remove redundant variables used for getaddrinfo(3).

Approved by: re (gjb)