sepgsql: Support for new post-ALTER access hook.

contrib/sepgsql/database.c

contrib/sepgsql/expected/alter.out

contrib/sepgsql/expected/ddl.out

contrib/sepgsql/hooks.c

contrib/sepgsql/proc.c

contrib/sepgsql/relation.c

contrib/sepgsql/schema.c

contrib/sepgsql/sepgsql.h

contrib/sepgsql/sql/alter.sql

contrib/sepgsql/sql/ddl.sql

contrib/sepgsql/test_sepgsql

doc/src/sgml/sepgsql.sgml