Bogus values for -D if SO_TIMESTAMPNS is unavailable

[sthen]

If the OS doesn't have SO_TIMESTAMPNS then fping chooses CLOCK_MONOTONIC for its timer if available. This results in bogus values for timestamps with fping -D as it uses an uptime-based clock rather than realtime.

$ fping -D -c1 127.0.0.1 2> /dev/null             
[596040.03537] 127.0.0.1 : [0], 64 bytes, 0.122 ms (0.122 avg, 0% loss)

This was introduced in 754a21e (

fping/src/fping.c

Line 127 in eac2034

#define CLOCKID CLOCK_MONOTONIC

) when SO_TIMESTAMP was replaced with SO_TIMESTAMPNS.

[sthen]

Oh, this is a duplicate of #203