pg_user cleanup.

src/backend/commands/user.c

src/backend/libpq/pg_hba.conf.sample

src/man/alter_user.l

src/man/catalogs.3

src/man/create_user.l

src/man/createuser.1

src/man/destroyuser.1

src/man/pg_dumpall.1

src/man/pg_hba.conf.5