Use callout(9) instead of timeout(9).

Reviewed by:	emax
Revert changes in r269180.  It could cause -c N option to enter an
infinite loop if no reply packet is received.

PR:	151023
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).

PR:	151023
Add unit tests for mkimg(1):