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

fusl/src/process/posix_spawn_file_actions_addopen.c

Index: fusl/src/process/posix_spawn_file_actions_addopen.c
diff --git a/fusl/src/process/posix_spawn_file_actions_addopen.c b/fusl/src/process/posix_spawn_file_actions_addopen.c
new file mode 100644
index 0000000000000000000000000000000000000000..368922c76b5259944059bad15d1c1d9d8e79fce5
--- /dev/null
+++ b/fusl/src/process/posix_spawn_file_actions_addopen.c
@@ -0,0 +1,20 @@
+#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;
+}