MXS-1300: Add link/unlink command to MaxCtrl

The link/unlink commands are similar to the add/remove commands in
MaxAdmin but they link services and monitors to a list of servers instead
of linking servers to a list of either services or monitors. This can
reduce the number of required calls to alter the configuration while
making the process easier for MaxCtrl to do via the REST API.

Also fixed a false negative where a service alteration which results in a
no-op would be counted as an error.

client/maxctrl/maxctrl.js

@@ -58,6 +58,8 @@ program
     .command(require('./lib/disable.js'))
     .command(require('./lib/create.js'))
     .command(require('./lib/destroy.js'))
+    .command(require('./lib/link.js'))
+    .command(require('./lib/unlink.js'))
     .help()
     .demandCommand(1, 'At least one command is required')
     .command('*', 'the default command', {}, () => {