mogadm: add "device next" sub-command

This is a convenience command to ease the process of adding devices
to a cluster.  This may be useful when there are many devices to be
added.  This provides no synchronization and multiple administrative
users will still have to coordinate device additions to avoid errors
on duplicates.

Adding synchronization would require extending the tracker
protocol, and is probably not worth the complexity.

mogadm

@@ -123,6 +123,9 @@ my $usage = {
                     "--weight=i" => "Positive numeric weight for device",
                 },
             },
+            next => {
+                des => "Show the next available devid.",
+            },
         },
     },
     domain => {
@@ -882,6 +885,21 @@ sub cmd_device_summary {
 
 }
 
+sub cmd_device_next {
+    my $devs = mogadm()->get_devices;
+    fail_text('no_devices') unless @$devs;
+    my $maxid = 0;
+    foreach my $dev (@$devs) {
+        my $devid = $dev->{devid};
+        if ($devid > $maxid) {
+            $maxid = $devid;
+        }
+    }
+
+    print($maxid + 1, "\n");
+    ok();
+}
+
 sub cmd_slave_list {
     my $mogadm = mogadm();
 
@@ -1474,6 +1492,8 @@ L</"HOST OPTIONS"> section.
 
 =item B<device list>
 
+=item B<device next>
+
 Functions for manipulating devices. For add and modify, device options are in
 the format of normal command line options and can include anything in the
 L</"DEVICE OPTIONS"> section.
@@ -1644,6 +1664,7 @@ Device manipulation:
     $ mogadm device list
     $ mogadm device summary
     $ mogadm device summary --status=dead,down
+    $ mogadm device next
     $ mogadm device add foo.local 16
     $ mogadm device add foo.local 17 --status=alive
     $ mogadm device mark foo.local 17 down