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

This enforces the "not nullptr" rule.

Prepare to make LightSong non-assignable.

Fixes search within mount points, resulting in error "No such
directory".

Closes #190

No longer a problem with NDK r16.

If a directory is a mount point, omit the "directory: " as well.

This bug is years old, but has become more visible now that mount
points are persistent in the state file.

If `SimpleDatabase::Visit` is called on a database that contains a mounted directry the URIs of the elements passed to the callbacks are not prefixed by the mountpoint path. This leads to lsinfo and add not working because they use the wrong URI. This pull request is using the `WalkMount` helper function to create prefixed versions of `VisitDirectory`, `VisitSong` and `VisitPlaylist` to add the correct prefix to the parameters of the callback functions.

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

Closes #169

assignment of `mnt->mounted_database` in `SimpleDatabase::Mount`.

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

Allows database plugins to use the EventThread, e.g. for CURL
integration.

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

An ino_t is usually a 64 bit integer, and some file systems (such as
Linux's kernel NFS client) really uses the upper 32 bit.  This can
lead to false positives in the directory loop detection in
FindAncestorLoop().  Increasing these two attributes (in
StorageFileInfo and Directory) to 64 bit adds little overhead, but
makes the check a lot safer.

SimpleDatabase::Visit() always throws exception when visiting a song.
Adding a song to the queue always results in a "No such directory"
error response, although the song is successfully added.  This
behaviour was introduced by commit
3ff728ab.  This patch fixes this.

https://bugs.musicpd.org/view.php?id=4601

Implementations shall use exceptions instead.