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

fusl/src/stdio/ungetc.c

Index: fusl/src/stdio/ungetc.c
diff --git a/fusl/src/stdio/ungetc.c b/fusl/src/stdio/ungetc.c
new file mode 100644
index 0000000000000000000000000000000000000000..180673a47663ad57b11d3e6525c35cc23858939a
--- /dev/null
+++ b/fusl/src/stdio/ungetc.c
@@ -0,0 +1,20 @@
+#include "stdio_impl.h"
+
+int ungetc(int c, FILE *f)
+{
+	if (c == EOF) return c;
+
+	FLOCK(f);
+
+	if (!f->rpos) __toread(f);
+	if (!f->rpos || f->rpos <= f->buf - UNGET) {
+		FUNLOCK(f);
+		return EOF;
+	}
+
+	*--f->rpos = c;
+	f->flags &= ~F_EOF;
+
+	FUNLOCK(f);
+	return c;
+}