Add min and max aggregates for inet/cidr data types.

doc/src/sgml/func.sgml

src/backend/utils/adt/network.c

src/include/catalog/catversion.h

src/include/catalog/pg_aggregate.h

src/include/catalog/pg_proc.h

src/include/utils/builtins.h

src/test/regress/expected/inet.out

src/test/regress/sql/inet.sql