add `db-select-first` to help confusing names

`db-scalar` returns a scalar value, `db-select-scalar` doesn't return a
scalar value. This is confusing. Add `db-select-first` to replace
`db-select-scalar`, since that's what this function actually does;
return the first element of the result set.

`db-select-scalar` is still in here to prevent breaking existing code,
but throws a deprecation warning.
Made db-select-scalar warning tell who is warning re ADWolf:#1201