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

fusl/src/stdio/fopen.c

+#include "stdio_impl.h"
+#include <fcntl.h>
+#include <string.h>
+#include <errno.h>
+
+FILE *fopen(const char *restrict filename, const char *restrict mode)
+{
+        FILE *f;
+        int fd;
+        int flags;
+
+        /* Check for valid initial mode character */
+        if (!strchr("rwa", *mode)) {
+                errno = EINVAL;
+                return 0;
+        }
+
+        /* Compute the flags to pass to open() */
+        flags = __fmodeflags(mode);
+
+        fd = sys_open(filename, flags, 0666);
+        if (fd < 0) return 0;
+        if (flags & O_CLOEXEC)
+                __syscall(SYS_fcntl, fd, F_SETFD, FD_CLOEXEC);
+
+        f = __fdopen(fd, mode);
+        if (f) return f;
+
+        __syscall(SYS_close, fd);
+        return 0;
+}
+
+LFS64(fopen);