ssh-agent: Don't have a timeout by default

IMHO, having a short timeout (1h) defeats the point of using
ssh-agent, which is not to have to retype passphrases all the time. Of
course, users who want timeouts can set programs.ssh.agentTimeout.

This restores the 14.04 behaviour.

nixos/modules/programs/ssh.nix

@@ -61,7 +61,8 @@ in
 
       agentTimeout = mkOption {
         type = types.nullOr types.string;
-        default = "1h";
+        default = null;
+        example = "1h";
         description = ''
           How long to keep the private keys in memory. Use null to keep them forever.
         '';