Add a "fork" of musl as //fusl.

fusl/src/thread/pthread_atfork.c

Index: fusl/src/thread/pthread_atfork.c
diff --git a/fusl/src/thread/pthread_atfork.c b/fusl/src/thread/pthread_atfork.c
new file mode 100644
index 0000000000000000000000000000000000000000..a40d7f63287079c2ce538360305b86e7174b06e9
--- /dev/null
+++ b/fusl/src/thread/pthread_atfork.c
@@ -0,0 +1,48 @@
+#include <pthread.h>
+#include "libc.h"
+
+static struct atfork_funcs {
+	void (*prepare)(void);
+	void (*parent)(void);
+	void (*child)(void);
+	struct atfork_funcs *prev, *next;
+} *funcs;
+
+static volatile int lock[2];
+
+void __fork_handler(int who)
+{
+	struct atfork_funcs *p;
+	if (!funcs) return;
+	if (who < 0) {
+		LOCK(lock);
+		for (p=funcs; p; p = p->next) {
+			if (p->prepare) p->prepare();
+			funcs = p;
+		}
+	} else {
+		for (p=funcs; p; p = p->prev) {
+			if (!who && p->parent) p->parent();
+			else if (who && p->child) p->child();
+			funcs = p;
+		}
+		UNLOCK(lock);
+	}
+}
+
+int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void))
+{
+	struct atfork_funcs *new = malloc(sizeof *new);
+	if (!new) return -1;
+
+	LOCK(lock);
+	new->next = funcs;
+	new->prev = 0;
+	new->prepare = prepare;
+	new->parent = parent;
+	new->child = child;
+	if (funcs) funcs->prev = new;
+	funcs = new;
+	UNLOCK(lock);
+	return 0;
+}