Add db-local user names, per discussion on hackers.

doc/src/sgml/runtime.sgml

src/backend/libpq/auth.c

src/backend/postmaster/postmaster.c

src/backend/utils/misc/guc.c

src/backend/utils/misc/postgresql.conf.sample

src/include/libpq/libpq-be.h

src/include/libpq/pqcomm.h