Relaxed system tables protection for cases UPDATE ... SET RDB$*_SOURCE = NULL. So far nullification is allowed for views/packages/procedures/functions/triggers only. I'm not sure somebody really needs hiding sources for defaults and check constraints, but this can be supported later, if required. This is a temporary solution, intended to be replaced with something more clever (e.g. special permissions or new DDL command) in v4.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@60183 65644016-39b1-43b1-bf79-96bc8fe82c15