std: Call pthread_atfork only once
std: Call pthread_atfork only once

Some libc implementations (glib) deduplicate identical hooks, others
(musl, macos) do not and blindly append them to an internal list.
Ensure there's only a single call to pthread_atfork to prevent unbounded
memory use when lots of threads/forks are used.

zig-lang/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig