transaction_isolation vs. default_transaction_isolation

Hackers,

A slew of settings in postgresql.conf, including work_mem, search_path,
DateStyle, and about 80 others are effectively just defaults for new
connections, since they can be changed by any user.

However, for *two* settings, and two settings only, we distinguish that
by naming an identical setting "default_*" in postgresql.conf.  This is
confusing and inconsistent with the rest of the GUCS.  Namely:

default_transaction_isolation
default_transaction_read_only
transaction_isolation
transaction_read_only

For 8.5, I would like to consolidate these into only 2 settings and drop
the default_* settings.

--Josh Berkus