Add FreeBSD support using the same API as on Linux

[tunedal]

It turns out FreeBSD uses the same API as Linux for this, so it just works.

At least it does on 13.3 and later. FreeBSD 13.2 and earlier need a small tweak (as described on the bug tracker), but since 13.2 is only supported until the end of June, I'm not sure if it's worth complicating the code. It works on 13.2 with this change to linux.py:

RTLD_DEFAULT = -2
libc = ctypes.CDLL("", handle=RTLD_DEFAULT)

But supporting 13.3 and later doesn't need any changes. In that case, is it confusing to have linux.py support both Linux and FreeBSD? Should I add a new freebsd.py that just imports the Linux implementation?

[WardBrian]

It seems OpenBSD may support the same API? https://man.openbsd.org/dl_iterate_phdr
And even Solaris: https://docs.oracle.com/cd/E88353_01/html/E37843/dl-iterate-phdr-3c.html

The lack of free github CI on these platforms makes it hard to be confident. In terms of naming, perhaps unix is the least-incorrect catchall?

[WardBrian]

Thanks!