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

+#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

src/linux/err.c

+#include <err.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+void vwarn(const char *fmt, va_list ap)
+{
+	vfprintf(stderr, fmt, ap);
+	perror("");
+}
+
+void vwarnx(const char *fmt, va_list ap)
+{
+	vfprintf(stderr, fmt, ap);
+	putc('\n', stderr);
+}
+
+void verr(int status, const char *fmt, va_list ap)
+{
+	vwarn(fmt, ap);
+	exit(status);
+}
+
+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);
+}
+
+void err(int status, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	verr(status, fmt, ap);
+	va_end(ap);
+}
+
+void errx(int status, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	verrx(status, fmt, ap);
+	va_end(ap);
+}