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

fusl/src/legacy/cuserid.c

Index: fusl/src/legacy/cuserid.c
diff --git a/fusl/src/legacy/cuserid.c b/fusl/src/legacy/cuserid.c
new file mode 100644
index 0000000000000000000000000000000000000000..4e78798ded26b09992bb69c4383447a805ae2ab9
--- /dev/null
+++ b/fusl/src/legacy/cuserid.c
@@ -0,0 +1,14 @@
+#define _GNU_SOURCE
+#include <pwd.h>
+#include <stdio.h>
+#include <unistd.h>
+
+char *cuserid(char *buf)
+{
+	struct passwd pw, *ppw;
+	long pwb[256];
+	if (getpwuid_r(geteuid(), &pw, (void *)pwb, sizeof pwb, &ppw))
+		return 0;
+	snprintf(buf, L_cuserid, "%s", pw.pw_name);
+	return buf;
+}