Handle empty argument to -b a little better and add it to usage()

Reference: /n/sources/patch/applied/con-baud
Date: Thu Mar  3 02:18:05 CET 2005

--- /sys/src/cmd/con/con.c	Wed Mar  2 23:00:22 2005
+++ /sys/src/cmd/con/con.c	Wed Mar  2 23:00:21 2005
@@ -61,7 +61,7 @@
 void
 usage(void)
 {
-	punt("usage: con [-drCvsn] [-l [user]] [-c cmd] net!host[!service]");
+	punt("usage: con [-drCvsn] [-b baud] [-l [user]] [-c cmd] net!host[!service]");
 }
 
 void
@@ -73,7 +73,7 @@
 	returns = 1;
 	ARGBEGIN{
 	case 'b':
-		baud = atoi(ARGF());
+		baud = atoi(EARGF(usage()));
 		break;
 	case 'd':
 		debug = 1;