add usage messages to auth/(enable disable status).

print useful message when user invokes these commands incorrectly
(thanks henesy)

sys/src/cmd/auth/disable

@@ -1,4 +1,10 @@
 #!/bin/rc
+
+if(! ~ $#* 1){
+	echo 'usage: auth/disable user' >[1=2]
+	exit usage
+}
+
 if(test -e /mnt/keys/$1)
 	echo -n disabled > /mnt/keys/$1/status
 if(test -e /mnt/netkeys/$1)

sys/src/cmd/auth/enable

@@ -1,4 +1,10 @@
 #!/bin/rc
+
+if(! ~ $#* 1){
+	echo 'usage: auth/enable user' >[1=2]
+	exit usage
+}
+
 if(test -e /mnt/keys/$1)
 	echo -n ok > /mnt/keys/$1/status
 if(test -e /mnt/netkeys/$1)

sys/src/cmd/auth/status

@@ -1,4 +1,10 @@
 #!/bin/rc
+
+if(! ~ $#* 1){
+	echo 'usage: auth/status user' >[1=2]
+	exit usage
+}
+
 cd /mnt/keys/$1 > /dev/null >[2=1] && {
 	stat=`{cat status}
 	exp=`{cat expire}