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.

The upper-case name seems to be the canonical one.

Move code from Path::RelativeFS() and make it generic.

Reduce overhead, don't duplicate the string.

There is no GetLengthUTF8() because strlen or std::string::size()
could be used instead.

- Move definition of SEPARATOR_UTF8 out of #ifdef
- Remove duplicated check in IsSeparatorUTF8