DERBY-6644: Support standard syntax for altering column nullability

Implement SQL:2011 syntax for altering nullability:

  ALTER TABLE t ALTER COLUMN c SET NOT NULL
  ALTER TABLE t ALTER COLUMN c DROP NOT NULL

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1607559 13f79535-47bb-0310-9956-ffa450edef68