Inconsistent semantics of -r and -s with one and many targets

Nobody has claimed this yet.

The code contains some `args->targets_len > 1` ifs that make no sense from the consistency viewpoint and have already tripped up the main user (cake-autorate). For example:

```
tsping -D -m -t -r 500 -s 0 127.0.0.1 127.0.0.2 # creates nicely half-second spaced pings to two targets
tsping -D -m -t -r 500 -s 0 127.0.0.1 127.0.0.2 127.0.0.3 # creates nicely half-second spaced pings to three targets
```

But, instead of creating nicely half-second spaced pings to one target, this creates an almost infinite-speed flood:

```
tsping -D -m -t -r 500 -s 0 127.0.0.1
```

Note that cake-autorate uses `--sleep-time "0" --target-spacing "${ping_response_interval_ms}"` as hard-coded `tsping` arguments, and will fall into this trap if the user sets `no_pingers` to 1.

Locate the command-line handling around the `args->targets_len > 1` conditions. Reproduce the one-target and multi-target commands from the issue, then trace how `--sleep-time` and `--target-spacing` affect pacing. Done means one target observes the same spacing semantics as multiple targets without an unintended flood.

Written by the indexing model from the issue text.