Thanks to some tricks I learned from Nan Liu and Dan Bode, I was
able to figure out a way to move all of the new version-related stuff
back into the params class, and clean up some of the if/_real stuff.
Basic tests for centos6 + pg 9.2 are passing.
Nan showed me a trick that will let us keep all of that param stuff
inside of params.pp, make it a parameterized class, and still support
the ability for users to specify a custom (non-system-default) pg
version. This commit takes the first step towards that pattern by
consolidating platform.pp and params.pp. (Everything old is new again!)
