backward-compatible. 2.0 was a substential rewrite. Main changes:
support some traditional ping features: loop mode, specify size of data
packets, specify how many pings to send, interpret ICMPs other than ICMP Echo
response.
make timings in tenths of milliseconds
do exponential backoff on retries
port to more systems
do better checking on whether received ICMP is for us
added -g option for generating IPs from a start to an end value
