WIP - Add ability to constrain backend temporary file space

Recently two systems here have suffered severely with excessive 
temporary file creation during query execution. In one case it could 
have been avoided by more stringent qa before application code release, 
whereas the other is an ad-hoc system, and err...yes.

In both cases it would have been great to be able to constrain the 
amount of temporary file space a query could use. In theory you can sort 
of do this with the various ulimits, but it seems pretty impractical as 
at that level all files look the same and you'd be just as likely to 
unexpectedly crippled the entire db a few weeks later when a table grows...

I got to wonder how hard this would be to do in Postgres, and attached 
is my (WIP) attempt. It provides a guc (max_temp_files_size) to limit 
the size of all temp files for a backend and amends fd.c cancel 
execution if the total size of temporary files exceeds this.

This is WIP, it does seem to work ok, but some areas/choices I'm not 
entirely clear about are mentioned in the patch itself. Mainly:

- name of the guc... better suggestions welcome
- datatype for the guc - real would be good, but at the moment the nice 
parse KB/MB/GB business only works for int

regards

Mark