BBR congestion control reports as CUBIC on FreeBSD

# Context

* Version of iperf3: iperf 3.17.1 (cJSON 1.7.15)
* Hardware: amd64
* Operating system: FreeBSD 14.1-RELEASE releng/14.1-n267679-10e31f0946d8 GENERIC
* Other relevant information: iperf3 was installed from FreeBSD packages

# Bug Report

## Steps to reproduce

use FreeBSD
enable the BBR TCP congestion control algorithm as described in [`tcp_bbr(4)`](https://man.freebsd.org/cgi/man.cgi?query=tcp_bbr)
execute `iperf3` with `--debug`
read the line beginning with `Congestion algorithm is`

## Expected behavior

```
Congestion algorithm is bbr
```

## Actual behavior

```
Congestion algorithm is cubic
```

## Possible Solution

> BBR is conceptually a CC, but in FreeBSD it is NOT implemented as a CC module. It is a TCP stack.

https://lists.freebsd.org/pipermail/freebsd-current/2020-April/075930.html

Here is some test code that demonstrates what I expect is the root of the confusion.

```c
#include
#include
#include
#include
#include
#include
#include

int
main(int argc, char **argv) {
char buf[256];
socklen_t len;

int s = socket(PF_INET, SOCK_STREAM, 6);
if (s < 0) err(1, "socket");

len = sizeof(buf);
if (getsockopt(s, IPPROTO_TCP, TCP_CONGESTION, buf, &len)) err(1, "getsockopt");
printf("tcp_congestion: %s\n", buf);

len = sizeof(buf);
if (getsockopt(s, IPPROTO_TCP, TCP_FUNCTION_BLK, buf, &len)) err(1, "getsockopt");
printf("tcp_function_blk: %s\n", buf);

close(s);
return 0;
}
```

output:

```
root@bert # ./getsockopt-tcp-function-blk
tcp_congestion: cubic
tcp_function_blk: bbr
```

This is only a minor bug, though it has caused confusion when dealing with other engineers who take the output from iperf as gospel.

Thank you for your work on iperf.