Improve err.h macros.

Handle 'NULL', emit the program name.

include/err.h

@@ -9,24 +9,46 @@
 
 #else
 
-#ifndef LIBCRYPTOCOMPAT_ERR_H
-#define LIBCRYPTOCOMPAT_ERR_H
+#ifndef LIBCOMPAT_ERR_H
+#define LIBCOMPAT_ERR_H
 
 #include <errno.h>
+#include <stdarg.h>
 #include <stdio.h>
 #include <string.h>
 
+extern char *__progname;
+
+static inline _warn(int err, const char *format, ...)
+	__attribute__ ((__format__ (__printf__, 2, 3)));
+
+static inline _warn(int err, const char *format, ...)
+{
+	va_list args;
+	va_start(args, format);
+	fprintf(stderr, "%s: ", __progname);
+	if (format != NULL) {
+		vfprintf(stderr, format, args);
+		if (err)
+			fprintf(stderr, ": ");
+	}
+	if (err)
+		fprintf(stderr, "%s", strerror(err));
+	fprintf(stderr, "\n");
+	va_end(args);
+}
+
 #define err(exitcode, format, args...) \
-  errx(exitcode, format ": %s", ## args, strerror(errno))
+  do { warn(format, ## args); exit(exitcode); } while (0)
 
 #define errx(exitcode, format, args...) \
   do { warnx(format, ## args); exit(exitcode); } while (0)
 
 #define warn(format, args...) \
-  warnx(format ": %s", ## args, strerror(errno))
+  _warn(errno, format, ## args)
 
 #define warnx(format, args...) \
-  fprintf(stderr, format "\n", ## args)
+  _warn(0, format, ## args)
 
 #endif