provide a better auth failure message.

Reference: /n/atom/patch/applied/cpuauthmsg
Date: Wed Feb 19 23:18:46 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/cpu.c	Wed Feb 19 23:18:32 2014
+++ /sys/src/cmd/cpu.c	Wed Feb 19 23:18:32 2014
@@ -374,11 +374,9 @@
 char*
 rexcall(int *fd, char *host, char *service)
 {
-	char *na;
-	char dir[MaxStr];
-	char err[ERRMAX];
-	char msg[MaxStr];
+	char *na, dir[MaxStr], err[ERRMAX], msg[MaxStr];
 	int n;
+	static char ret[128];
 
 	na = netmkaddr(host, 0, service);
 	procsetname("dialing %s", na);
@@ -404,8 +402,10 @@
 	/* authenticate */
 	procsetname("%s: auth via %s", origargs, am->name);
 	*fd = (*am->cf)(*fd);
-	if(*fd < 0)
-		return "can't authenticate";
+	if(*fd < 0){
+		snprint(ret, sizeof ret, "can't authenticate: %s", am->name);
+		return ret;
+	}
 	return 0;
 }