Outdated non-POSIX FreeBSD API used by tests

[ctrlcctrlv]

setgrent returned an int two years ago, but now (as of freebsd/freebsd-src@786c392 ) returns void. We're testing against the outdated API, which became an issue in #934

Thanks to @gnzlbg for finding the issue.

[bdrewery]

Note that setgrent did not get a new symbol version in FreeBSD for the changed ABI.

[gnzlbg]

@asomers IIUC this function still returns an int in FreeBSD 11, that is, we can't change its prototype to specify that it does not return anything for targets that should also work in FreeBSD11.

In FreeBSD12, the return value is invalid, and creating an invalid value is UB in Rust. I agree that, as long as LTO between Rust code and FreeBSD's system library is not possible, this UB cannot break code that just ignores the return value.