dmstats: fix interval number reporting with --count=0

dmstats: fix interval number reporting with --count=0

When --count=0 interval numbers are miscalculated:

Interval     #18446744069414584325     time delta:    999920887ns
Interval     #18446744069414584325   current err:       -79113ns
End interval #18446744069414584325  duration:    999920887ns

This is because the command line argument is cast through the
uint32_t type, and fixed to UINT32_MAX:

  _count = ((uint32_t)_int_args[COUNT_ARG]) ? : UINT32_MAX;

We also need to handle --count=0 specially when calculating the
interval number: since intervals count from #1, this must account
for the implicit "minus one" when converting from zero to the
UINT64_MAX value used (which is too large to store in _int_args).