authenticate: replace xclip whith xsel

xclip does not close stdout. This broke the call of

	eval `./authenticate`

See astrand/xclip#20 for details.

misc/README.md

@@ -15,7 +15,7 @@ E.g. to fetch all elements, you can run
 	./authorize elements
 
 ### Clipboard
-If `xclip` is present on the system, `./authenticate` will copy the JWT token to the
+If `xsel` is present on the system, `./authenticate` will copy the JWT token to the
 clipboard.
 
 ## Generate TLS key pairs

misc/authenticate

@@ -53,7 +53,7 @@ fi
 echo export "'"TOKEN=$token"'"
 echo export "'"VEOURL=$url"'"
 
-if which xclip >/dev/null 2>&1
+if which xsel >/dev/null 2>/dev/null
 then
-	echo $TOKEN | xclip -selection clipboard
+	echo -n "$TOKEN" | xsel -i -b
 fi