Create a new GUC variable search_path to control the namespace search

Create a new GUC variable search_path to control the namespace search
path.  The default behavior if no per-user schemas are created is that
all users share a 'public' namespace, thus providing behavior backwards
compatible with 7.2 and earlier releases.  Probably the semantics and
default setting will need to be fine-tuned, but this is a start.