Do a pass of code review for the ALTER TABLE ADD INHERITS patch. Keep

Do a pass of code review for the ALTER TABLE ADD INHERITS patch. Keep
the read lock we hold on the table's parent relation until commit.
Update equalfuncs.c for the new field in AlterTableCmd. Various
improvements to comments, variable names, and error reporting.

There is room for further improvement here, but this is at least
a step in the right direction.