Open or create database at path given by *file*.
Optional argument *flag* can be 'r' (default) for read-only access, 'w'
for read-write access of an existing database, 'c' for read-write access
to a new or existing database, and 'n' for read-write access to a new
database.
Note: 'r' and 'w' fail if the database doesn't exist; 'c' creates it
only if it doesn't exist; and 'n' always creates a new database.