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

fusl/src/process/posix_spawn_file_actions_addopen.c

+#include <spawn.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include "fdop.h"
+
+int posix_spawn_file_actions_addopen(posix_spawn_file_actions_t *restrict fa, int fd, const char *restrict path, int flags, mode_t mode)
+{
+        struct fdop *op = malloc(sizeof *op + strlen(path) + 1);
+        if (!op) return ENOMEM;
+        op->cmd = FDOP_OPEN;
+        op->fd = fd;
+        op->oflag = flags;
+        op->mode = mode;
+        strcpy(op->path, path);
+        if ((op->next = fa->__actions)) op->next->prev = op;
+        op->prev = 0;
+        fa->__actions = op;
+        return 0;
+}