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

fusl/src/process/fork.c

+#include <unistd.h>
+#include <string.h>
+#include <signal.h>
+#include "syscall.h"
+#include "libc.h"
+#include "pthread_impl.h"
+
+static void dummy(int x)
+{
+}
+
+weak_alias(dummy, __fork_handler);
+
+pid_t fork(void)
+{
+        pid_t ret;
+        sigset_t set;
+        __fork_handler(-1);
+        __block_all_sigs(&set);
+#ifdef SYS_fork
+        ret = syscall(SYS_fork);
+#else
+        ret = syscall(SYS_clone, SIGCHLD, 0);
+#endif
+        if (!ret) {
+                pthread_t self = __pthread_self();
+                self->tid = __syscall(SYS_gettid);
+                self->robust_list.off = 0;
+                self->robust_list.pending = 0;
+                libc.threads_minus_1 = 0;
+        }
+        __restore_sigs(&set);
+        __fork_handler(!ret);
+        return ret;
+}