plpgsql - additional extra checks

Hi

I am starting new thread for this patch (related to merging some ideas from
plpgsql2 project thread).

Here is simple patch with two new extra_warnings, extra_errors checks

1. strict_multi_assignment - checks the number of target variables and
source values.
2. too_many_rows - checks if returned rows is more than one

The extra checks are designed to help identify some possible errors or
issues. It is not way how to change a design, behave and features of
plpgsql language.

Now, the extra checks are three fields only - it is easy maintainable now,
and I don't see a necessity to implement some another management for extra
checks.

Regards

Pavel