Some small issues with `su` replacement

[lukebakken]

Related to #125 and #72

FreeBSD 9.2 test VM:

• riak command can be run to get usage without sudo, riak-admin cannot:

  $ riak
  Usage: riak «command»
  where «command» is one of the following:
    { help | start | stop | restart | reboot | ping | console | attach
    attach-direct | ertspath | chkconfig | escript | version | getpid
    top [-interval N] [-sort { reductions | memory | msg_q }] [-lines N] } |
    config { generate | effective | describe VARIABLE } [-l debug]
  
  Run `riak help` for more detailed information.
  
  $ riak-admin
  You need to be root or use sudo to run this command

• riak ping requires sudo (fixed already I believe)

[Vagabond]

Jared and I discussed the riak-admin thing, but since riak-admin can ONLY be run with superuser privs (or as the riak user) anyway, we decided it wasn't terribly interesting to print the usage message.

[lukebakken]

OK!