make err.h functions print __progname

patch by Strake. previously is was not feasible to duplicate this
functionality of the functions these were modeled on, since argv[0]
was not saved at program startup, but now that it's available it's
easy to use.

src/legacy/err.c

@@ -3,14 +3,18 @@
 #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);
 	perror("");
 }
 
 void vwarnx(const char *fmt, va_list ap)
 {
+	fprintf (stderr, "%s: ", __progname);
 	if (fmt) vfprintf(stderr, fmt, ap);
 	putc('\n', stderr);
 }