bug: reject non-numeric arguments to -c

Arguments to -c wasn't error checked. Accept only numerical arguments.
For overflow and underflow accept the "clamped" values.
main: target_probe should determine sequence number

Let xping.c's target_probe determine sequence number to be used for
current transmission. Thus reducing t->npkts use in modules.
icmp-unpriv: attempt to register transmit errors

Transmit errors are given without sequence number, assume they refer to
the most recent transmitted packet.
icmp-unpriv: catch timing drifts

There is a disconnect between xping.c triggers and the forked off ping
program. For all non-recent replies determine if ping has drifted too
far. A non recent reply is at least i_interval old, but if ping doesn't
think the reply's rtt is at least i_interval there is drift.

Then reset the probe and leave one reply dangling, which will be
registered as a transmit error.
main: removed debug flag from linking phase