instead of an "i/o count too small", detect a missing capability (empty, null string)
before the write, and diagnose it as such.

Reference: /n/sources/patch/applied/chuid_empty
Date: Thu Apr 11 00:23:49 CES 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/libauth/auth_chuid.c	Thu Apr 11 00:22:58 2013
+++ /sys/src/libauth/auth_chuid.c	Thu Apr 11 00:22:56 2013
@@ -10,7 +10,7 @@
 {
 	int rv, fd;
 
-	if(ai == nil || ai->cap == nil){
+	if(ai == nil || ai->cap == nil || *ai->cap == 0){
 		werrstr("no capability");
 		return -1;
 	}