Make 'sqlite3:' URL paths relative again

This returns to the behaviour from 4.0.

Use an extra (fourth!) slash for an absolute path:

    sqlite3:////some/full/path

Reverts fbb79b517f3127ba620fedd01849f9628b78d6ce and dd93a5f4598b420710fb4a9a2628abfd98799fb6; fixes #14495.