Current POSIX standard requires fork() to be async-signal safe. Neither our implementation, nor implementations in other operating systems are, and practically it is impossible to make fork() async-signal safe without too much efforts. Also, that could put undue requirement that all atfork handlers should be async-signal safe as well, which contradicts its main use.
As result, Austin Group dropped the requirement, and added a new function _Fork() that should be async-signal safe, but it does not call atfork handlers. Basically, _Fork() can be implemented as a raw syscall.
Release of glibc 2.34 added _Fork(), do the same for FreeBSD. Clarify threading behavior for fork() in the manpage.