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.
Add exit codes when failing.

On mogdelete, mogfetch and moglistkeys when failing, it writes to STDERR
with there's an error, but it exits with 0. Change it to exist with an
error.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Checking in changes prior to tagging of version 2.29.

Changelog diff is:

diff --git a/Changes b/Changes
index c3bb0e6..6e65563 100644
--- a/Changes
+++ b/Changes
@@ -1,3 +1,9 @@
+2.29 -- 2014-12-15
+
+   * Add exit codes when failing. (Jesus Roncero <jesus@we7.com>)
+
+   * mogadm: add "device next" sub-command (Eric Wong <normalperson@yhbt.net>)
+
 2.28 -- 2013-08-07

    * mogadm: improve "device list" formatting (Eric Wong <normalperson@yhbt.net>)