ntp: fix presend in interleaved mode

ntp: fix presend in interleaved mode

The presend option in interleaved mode uses two presend requests instead
of one to get an interleaved response from servers like chrony which
delay the first interleaved response due to an optimization saving
timestamps only for clients actually using the interleaved mode.

After commit 0ae6f2485b97 ("ntp: don't use first response in interleaved
mode") the first interleaved response following the two presend
responses in basic mode is dropped as the preferred set of timestamps
minimizing error in delay was already used by the second sample in
basic mode. There are only three responses in the burst and no sample is
accumulated.

Increasing the number of presend requests to three to get a fourth
sample would be wasteful. Instead, allow reusing timestamps of the
second presend sample in basic mode, which is never accumulated.

Reported-by: Aaron Thompson
Fixes: 0ae6f2485b97 ("ntp: don't use first response in interleaved mode")