Add GUC setting for Australian timezones. Uses new GUC boolean callback

doc/src/sgml/runtime.sgml

src/backend/utils/adt/datetime.c

src/backend/utils/misc/guc.c

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

src/include/utils/datetime.h

src/include/utils/guc.h

src/test/regress/expected/horology-no-DST-before-1970.out

src/test/regress/expected/horology-solaris-1947.out

src/test/regress/expected/horology.out

src/test/regress/expected/timestamp.out

src/test/regress/sql/horology.sql

src/test/regress/sql/timestamp.sql