pmrep: allow sub-second sampling intervals

pmrep: allow sub-second sampling intervals

There's no particular reason to forbid interval being less than
one second for pmrep(1), the only reason preventing it were some
internal calculations using int().

Lift the restriction and update the code as needed. Add some QA.

Interval 0 would work as well but I guess it's best to forbid it,
hard to see any realistic use cases for it (and something like
-t 0.1 should suffice just fine in any case).

Note that in some cases the external header's end time now may
differ from what will be the actual end time (interval and
samples still being correct), but I think the approach below
is the most reasonable one, it shouldn't surprise users.