SET NOT NULL/DROP NOT NULL patch

This is a complete patch to implement changing the nullability of an
attribute. It passes all regressions tests. It includes its own quite
comprehensive regression test suite and documentation. It prevents you from
modifying system tables, non-table relations, system attributes, primary
keys and columns containing NULLs. It fully supports inheritance. I have
made some small changes to TODO to reflect this new functionality, plus
corrected some other TODO items.

The only thing I haven't checked are my ecpg changes. I would like someone
with more ecpg experience to check my preproc.y changes.

Please consider for 7.3!

Since I have now added two new large functions to command.c, I propose that
sometime before 7.3 beta, command.c is refactored and an alter.c created.
There is lots of common code in the Alter* functions that should be reused.

Chris