[PATCH] pg_dump: Sort overloaded functions in deterministic order

I renamed the new element to DumpableObject from "proargs" to the more
general name "sortkey".

This way this element can be used by any object types in the future,
which might require sorting by additional information than type, namespace
and name.

Currently, it's only set for functions/aggregates though, its NULL for all
other object types.

I felt less ugly to add a new element with a general name than one specific
for functions.

I also moved the check to the last part of DOTypeNameCompare, just before
sorting by OIDs as a last resort.

Feedback on the implementation is welcomed.

If this can be achieved without adding a new element to DumpableObject,
it is of course much better, but I couldn't find a way of doing that.