Allow units to be specified with configuration settings.

doc/src/sgml/catalogs.sgml

doc/src/sgml/config.sgml

src/backend/catalog/system_views.sql

src/backend/utils/misc/guc-file.l

src/backend/utils/misc/guc.c

src/bin/initdb/initdb.c

src/include/utils/guc_tables.h

src/test/regress/expected/rules.out