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

fusl/src/legacy/err.c

Index: fusl/src/legacy/err.c
diff --git a/fusl/src/legacy/err.c b/fusl/src/legacy/err.c
new file mode 100644
index 0000000000000000000000000000000000000000..0d6ab5242d9b769b31ced7fcc1a62fd5273a402c
--- /dev/null
+++ b/fusl/src/legacy/err.c
@@ -0,0 +1,67 @@
+#include <err.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+extern char *__progname;
+
+void vwarn(const char *fmt, va_list ap)
+{
+	fprintf (stderr, "%s: ", __progname);
+	if (fmt) {
+		vfprintf(stderr, fmt, ap);
+		fputs (": ", stderr);
+	}
+	perror(0);
+}
+
+void vwarnx(const char *fmt, va_list ap)
+{
+	fprintf (stderr, "%s: ", __progname);
+	if (fmt) vfprintf(stderr, fmt, ap);
+	putc('\n', stderr);
+}
+
+_Noreturn void verr(int status, const char *fmt, va_list ap)
+{
+	vwarn(fmt, ap);
+	exit(status);
+}
+
+_Noreturn void verrx(int status, const char *fmt, va_list ap)
+{
+	vwarnx(fmt, ap);
+	exit(status);
+}
+
+void warn(const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	vwarn(fmt, ap);
+	va_end(ap);
+}
+
+void warnx(const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	vwarnx(fmt, ap);
+	va_end(ap);
+}
+
+_Noreturn void err(int status, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	verr(status, fmt, ap);
+	va_end(ap);
+}
+
+_Noreturn void errx(int status, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	verrx(status, fmt, ap);
+	va_end(ap);
+}