pgsql: Add an option to AlterTableCreateToastTable() to allow its caller

Log Message:
-----------
Add an option to AlterTableCreateToastTable() to allow its caller to force
a toast table to be built, even if the sum-of-column-widths calculation
indicates one isn't needed.  This is needed by pg_migrator because if the
old table has a toast table, we have to migrate over the toast table since
it might contain some live data, even though subsequent column drops could
mean that no recently-added rows could require toasting.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        toasting.c (r1.14 -> r1.15)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/toasting.c?r1=1.14&r2=1.15)
    pgsql/src/backend/commands:
        cluster.c (r1.183 -> r1.184)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c?r1=1.183&r2=1.184)
        tablecmds.c (r1.281 -> r1.282)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.281&r2=1.282)
    pgsql/src/backend/executor:
        execMain.c (r1.323 -> r1.324)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/execMain.c?r1=1.323&r2=1.324)
    pgsql/src/backend/tcop:
        utility.c (r1.306 -> r1.307)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c?r1=1.306&r2=1.307)
    pgsql/src/include/catalog:
        toasting.h (r1.6 -> r1.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/toasting.h?r1=1.6&r2=1.7)