c++ broker authentication

[Gordon Sim]

Thanks to Matt Farrellee, the c++ broker now has initial support for 
PLAIN authentication (r647716). This is optional at compile time and 
runtime and is based on the cyrus sasl lib (see the docs for that 
library for info on configuring etc for your platform).

The runtime option is --auth (yes|no) and will eventually be turned on 
by default (when support for authentication has been enabled at compile 
time). However as this may well break any automated tests or other 
scripts, I have temporarily left it off by default to allow a short time 
to update any such scripts.

If you have automated tests or applications running against the c++ 
broker in an environment where you don't want to configure for 
authentication, you should add '--auth no' to the options you pass to 
the broker (be it on the command line or through conf file/env variables).

Unless there is a convincing reason not to, I will turn on 
authentication by default next week.