Even if the LocalStorage is available, return the "udisks://" URI when
the MapUTF8() parameter is an empty string.  This fixes the mount URI
in the state file.

Closes #487

Fixes the "org.freedesktop.UDisks2.Error.AlreadyMounted" error.

Closes #485

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.

The absolute udisks:// URI is not usable with InputStream::Open(), and
thus we need to return the LocalStorage::MapUTF8() return value
instead.

Documentation will follow soon.