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

fusl/src/stdio/__fmodeflags.c

Index: fusl/src/stdio/__fmodeflags.c
diff --git a/fusl/src/stdio/__fmodeflags.c b/fusl/src/stdio/__fmodeflags.c
new file mode 100644
index 0000000000000000000000000000000000000000..da9f23b633a0828a07cad171318d5d05132c2827
--- /dev/null
+++ b/fusl/src/stdio/__fmodeflags.c
@@ -0,0 +1,16 @@
+#include <fcntl.h>
+#include <string.h>
+
+int __fmodeflags(const char *mode)
+{
+	int flags;
+	if (strchr(mode, '+')) flags = O_RDWR;
+	else if (*mode == 'r') flags = O_RDONLY;
+	else flags = O_WRONLY;
+	if (strchr(mode, 'x')) flags |= O_EXCL;
+	if (strchr(mode, 'e')) flags |= O_CLOEXEC;
+	if (*mode != 'r') flags |= O_CREAT;
+	if (*mode == 'w') flags |= O_TRUNC;
+	if (*mode == 'a') flags |= O_APPEND;
+	return flags;
+}