Closes https://github.com/MusicPlayerDaemon/MPD/issues/607

Prepare for templated functions.

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.

Some database plugins don't use a Storage (e.g. UPnP), and with this
plugin, DatabaseDetachSong() can crash.

Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Enables using range-based "for".

Build the UTF-8 version of the path automatically in the constructor.

Replaces map_to_relative_path() from Mapper.cxx.

Use Storage::MapUTF8() internally, don't use global variables.

Move from db/DatabaseGlue.cxx, eliminating global variable.

Reduce global variable usage, move to frontend code.

There was quite a lot of duplicate code for loading DetachedSong
objects, with different semantics for "securely" loading local files.

Reduce bloat.