support sql server with migrations

Since `clsql:create-table` has no way to specify that a column should
be an unbounded text field, we have to specify that manually. The
previous implementation used `longchar`, which isn't referenced
anywhere in clsql so I'm not sure where that came from.
Merge branch 'master' of git.acceleration.net:/var/git/clsql-helper
support nil as a valid value to search for in by-col
minor code de-duplication
db-select-scalar should be setting limit to 1 rather than flatp
Merge branch 'master' of git:/var/git/clsql-helper