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

fusl/src/process/posix_spawn_file_actions_adddup2.c

Index: fusl/src/process/posix_spawn_file_actions_adddup2.c
diff --git a/fusl/src/process/posix_spawn_file_actions_adddup2.c b/fusl/src/process/posix_spawn_file_actions_adddup2.c
new file mode 100644
index 0000000000000000000000000000000000000000..0367498fd4f36ca9fe5d0ae12ec65fc15936bc5b
--- /dev/null
+++ b/fusl/src/process/posix_spawn_file_actions_adddup2.c
@@ -0,0 +1,17 @@
+#include <spawn.h>
+#include <stdlib.h>
+#include <errno.h>
+#include "fdop.h"
+
+int posix_spawn_file_actions_adddup2(posix_spawn_file_actions_t *fa, int srcfd, int fd)
+{
+	struct fdop *op = malloc(sizeof *op);
+	if (!op) return ENOMEM;
+	op->cmd = FDOP_DUP2;
+	op->srcfd = srcfd;
+	op->fd = fd;
+	if ((op->next = fa->__actions)) op->next->prev = op;
+	op->prev = 0;
+	fa->__actions = op;
+	return 0;
+}