whoops, use argv0 instead.

Notes:
Sat Jan 28 09:32:00 EST 2006 rsc
    In general I dislike programs to use argv0 for warnings.
    It's fine usually, but then when you invoke a program
    as /386/bin/htmlroff or something like that, I don't 
    need to see that in the error messages.
    


Reference: /n/sources/patch/sorry/htmlroff-warn2
Date: Sat Jan 28 12:43:18 CET 2006
Reviewed-by: rsc

--- /sys/src/cmd/htmlroff/util.c	Sat Jan 28 12:42:51 2006
+++ /sys/src/cmd/htmlroff/util.c	Sat Jan 28 12:42:49 2006
@@ -78,7 +78,7 @@
 {
 	va_list arg;
 	
-	fprint(2, "%L: ");
+	fprint(2, "%s: %L: ", argv0);
 	va_start(arg, fmt);
 	vfprint(2, fmt, arg);
 	va_end(arg);