Merge ALTER GROUP ADD/DROP rules by creating add_drop action.

src/backend/parser/gram.y

src/backend/utils/adt/mac.c

src/test/regress/data/agg.data

src/test/regress/data/constrf.data

src/test/regress/data/constro.data

src/test/regress/data/desc.data

src/test/regress/data/emp.data

src/test/regress/data/hash.data

src/test/regress/data/onek.data

src/test/regress/data/person.data

src/test/regress/data/real_city.data

src/test/regress/data/rect.data

src/test/regress/data/streets.data

src/test/regress/data/stud_emp.data

src/test/regress/data/student.data

src/test/regress/data/tenk.data