[src] Fix sample rounding errors in extract-segments (#3321)

With a segments file constructed from exact wave file durations
some segments came out one sample short. The reason is the
multiplication of the float sample frequency and double audio
time point is inexact. For example, float 8000.0 multiplied
by double 2.03 yields 16239.99999999999, one LSB short of the
correct sample number 16240.

Also changed all endpoint calculations so that they performed
in seconds, not sample numbers, as this does not require a
conversion in nearly every comparison, and report positions in
diagnostic messages also in seconds, not sample numbers.