pgsql: Add more checks against altering typed tables - Prohibit

Log Message:
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Add more checks against altering typed tables

- Prohibit altering column type
- Prohibit changing inheritance
- Move checks from Exec to Prep phases in ALTER TABLE code

backpatched to 9.0

Modified Files:
--------------
pgsql/src/backend/commands:
tablecmds.c (r1.332 -> r1.333)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.332&r2=1.333)
pgsql/src/test/regress/expected:
typed_table.out (r1.1 -> r1.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/typed_table.out?r1=1.1&r2=1.2)
pgsql/src/test/regress/sql:
typed_table.sql (r1.1 -> r1.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/typed_table.sql?r1=1.1&r2=1.2)