Contrib -- PostgreSQL shared variables

This is a first pass on a simple shared memory variable system for
PostgreSQL. I would appriciate anyone interested in this functionality to
rip it apart.

It basically adds this functionality:

SetShared('name', value);
GetSharedInt('name');
SetSharedText('name);
RemovedShared('name');

I also added two extra functions that are sort of a kludge, but could be
very helpful.

AddSharedInt('name', value);
SubSharedInt('name', value);

These add or subtect the 'value' from the variable and return the result.