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.

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

It was used in a wrong way, which did not deal with errors
consistently.  And if that's wrong, there is no need for FindStorage()
at all - let's remove it and the confusion around it.

"override" implies "virtual".

This is the backend for the upcoming "mount" command.