Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Modeled after std::filesystem::operator/() from C++17.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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.