Closes https://github.com/MusicPlayerDaemon/MPD/issues/1048

Found with cppcoreguidelines-interfaces-global-init
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Closes https://github.com/MusicPlayerDaemon/MPD/issues/841

The `db->close()` call was a `nullptr` dereference because the `db`
variable had already been moved.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/839

Reduces unstripped size. stripped size is the same.

Also took the time to remove using std::placeholders.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Eliminates one strlen() call.

Use std::string_view to avoid modifying the string for the temporary
null terminators.

This method gets called a lot during MPD startup, via FindChild() and
directory_load_subdir(), so this is worth optimizing at the expense of
code readability.

This speeds up MPD startup by 10%.

Found with modernize-use-default-member-init
Signed-off-by: Rosen Penev <rosenp@gmail.com>

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former is deprecated by C++14. It's also functionally the same.

From the standard:

19.4
The header<cerrno>is described in Table 43. Its contents are the same as
the POSIX header<errno.h>,except that errno shall be defined as a macro.
[Note: The intent is to remain in close alignment with the POSIX
standard.] A separate errno value shall be provided for each thread.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The former was deprecated with C++14.

According to the C++11 and C++17 standards, both files are identical.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with performance-unnecessary-value-param
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with modernize-make-unique
Signed-off-by: Rosen Penev <rosenp@gmail.com>

This commit adds a PlaylistPlugin attribute "as_folder" which for now
is only enabled in the "CUE" playlist plugin (which handles separate
"*.cue" files).  If a playlist with this flag set is being scanned
during database update, it will be parsed and its contents will be
added to the database.  This allows clients to inspect them like
directories and its contents will be searchable.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/39

Will be used for Song objects representing tracks inside a CUE file.

Eliminate unnecessary dynamic allocations.

This attribute is not a URI; it is just the filename without its
parent directory path.  To avoid confusion, let's rename it to
"filename", leaving the struct without a "uri" attribute.