support the nonstandard err.h interfaces

note that unlike the originals, these do not print the program
name/argv[0] because we have not saved it anywhere. this could be
changed in __libc_start_main if desired.

include/err.h

@@ -0,0 +1,24 @@
+#ifndef _ERR_H
+#define _ERR_H
+
+#include <stdarg.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void warn(const char *, ...);
+void vwarn(const char *, va_list);
+void warnx(const char *, ...);
+void vwarnx(const char *, va_list);
+
+void err(int, const char *, ...);
+void verr(int, const char *, va_list);
+void errx(int, const char *, ...);
+void verrx(int, const char *, va_list);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif