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

fusl/src/process/fork.c

Index: fusl/src/process/fork.c
diff --git a/fusl/src/process/fork.c b/fusl/src/process/fork.c
new file mode 100644
index 0000000000000000000000000000000000000000..b96f0024f4c03c86e8ac6d72b4305576d0a020c6
--- /dev/null
+++ b/fusl/src/process/fork.c
@@ -0,0 +1,35 @@
+#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;
+}