Add _Fork()

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 would 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.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D31378