Cause initdb to create a third standard database "postgres", which

unlike template0 and template1 does not have any special status in
terms of backend functionality.  However, all external utilities such
as createuser and createdb now connect to "postgres" instead of
template1, and the documentation is changed to encourage people to use
"postgres" instead of template1 as a play area.  This should fix some
longstanding gotchas involving unexpected propagation of database
objects by createdb (when you used template1 without understanding
the implications), as well as ameliorating the problem that CREATE
DATABASE is unhappy if anyone else is connected to template1.
Patch by Dave Page, minor editing by Tom Lane.  All per recent
pghackers discussions.