The new class is pluggable, to prepare for gzipped database files.

For now, the TextFile class remains, and will be refactored away
later.

Eliminate the std::string overhead.

The XDG cache directory is part of the "base directories" spec like
$XDG_CONFIG_HOME, not "user directories".

Move code from CreateConfiguredDatabase() and add XDG support.  This
implements an automatic Linux fallback for the setting "db_file" if
none was specified.