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

fusl/src/legacy/err.c

+#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);
+}