Fixed CORE-4464: Duplicate tags for CREATE/ALTER USER not handled correctly

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59758 65644016-39b1-43b1-bf79-96bc8fe82c15
Avoid non-standard use of RDB$ROLES.RDB$SYSTEM_FLAG

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@59759 65644016-39b1-43b1-bf79-96bc8fe82c15