Fill in the remainder of the documented functionality for
n_d_open_with_config with respect to config file location. Similar
searching default locations of the database file still needs to be
added.
The main goal is to allow configuration information to be temporarily
overridden by a separate config file. That will require further
changes not in this commit.
The performance impact is unclear, and will depend on the balance
between number of queries and number of distinct metadata items read
on the first call to n_d_get_config.
