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.

Signed-off-by: FlashSystems <developer@flashsystems.de>

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

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

Save the state file 2 minutes after the last change.  This reduces the
disruptions by an idle MPD, and MPD can be paged out permanently until
it is used.

This is the beginning of multi-player support.  There will be support
for multiple Partition objects in one MPD process.

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.