This doesn't make sense on Android; there is no home directory for
users.

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.