dpctl: Fix jump through wild pointer in "dpctl/help".

dpctl: Fix jump through wild pointer in "dpctl/help".

dpctl_unixctl_handler() didn't fully initialize the dpctl_params structure
it passed to the handler, which meant that dpctl_help() could see a nonnull
(indeterminate) 'usage' pointer and jump through it, causes a crash.
This commit fixes the crash by fully initializing the structure.

The dpctl/help command wasn't going to do anything useful anyway, so this
commit also stops registering it.

Reported-by: Murali R <muralirdev@gmail.com>
Reported-at: http://openvswitch.org/pipermail/discuss/2015-October/019135.html
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>