Support SQL-compliant triggers on columns, ie fire only if certain columns

Support SQL-compliant triggers on columns, ie fire only if certain columns
are named in the UPDATE's SET list.

Note: the schema of pg_trigger has not actually changed; we've just started
to use a column that was there all along.  catversion bumped anyway so that
this commit is included in the history of potentially interesting changes
to system catalog contents.

Itagaki Takahiro