err: Add err(), warn(), errx() and warnx() familiy of functions

Some systems such as Windows or musl-libc based ones do not have these
BSD extensions. In addition libbsd itself is making use of the warnx()
functions, so we better provide these interfaces in case they are
missing.

src/err.c

@@ -26,6 +26,9 @@
  */
 
 #include <err.h>
+#ifdef LIBBSD_NEED_ERR_H_FUNCS
+#include <errno.h>
+#endif
 #include <string.h>
 #include <stdarg.h>
 #include <stdlib.h>
@@ -73,3 +76,76 @@ errc(int status, int code, const char *format, ...)
 	verrc(status, code, format, ap);
 	va_end(ap);
 }
+
+#ifdef LIBBSD_NEED_ERR_H_FUNCS
+void
+vwarn(const char *format, va_list ap)
+{
+	vwarnc(errno, format, ap);
+}
+
+void
+warn(const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	vwarnc(errno, format, ap);
+	va_end(ap);
+}
+
+void
+vwarnx(const char *format, va_list ap)
+{
+	fprintf(stderr, "%s: ", getprogname());
+	if (format)
+		vfprintf(stderr, format, ap);
+	fprintf(stderr, "\n");
+}
+
+void
+warnx(const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	vwarnx(format, ap);
+	va_end(ap);
+}
+
+void
+verr(int status, const char *format, va_list ap)
+{
+	verrc(status, errno, format, ap);
+}
+
+void
+err(int status, const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	verrc(status, errno, format, ap);
+	va_end(ap);
+}
+
+void
+verrx(int eval, const char *format, va_list ap)
+{
+	fprintf(stderr, "%s: ", getprogname());
+	if (format)
+		vfprintf(stderr, format, ap);
+	fprintf(stderr, "\n");
+	exit(eval);
+}
+
+void
+errx(int eval, const char *format, ...)
+{
+	va_list ap;
+
+	va_start(ap, format);
+	verrx(eval, format, ap);
+	va_end(ap);
+}
+#endif