FR: add local receive timestamp to --icmp-timestamp

[auerswal]

@moeller0 asked for the addition of a local receive timestamp value to the output from --icmp-timestamp in #353 (comment). The existing -D, --timestamp option requires loop or count mode, and --timestamp-format does not provide the RFC 792 format of milliseconds since midnight UTC. Thus the output of --icmp-timestamp should be amended similarly to @moeller0's suggestion.

I too think that something like this would be useful for tracking changes in one way delay.