[[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>

Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

Closes https://github.com/MusicPlayerDaemon/MPD/issues/234

implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.

For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.

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

Prepare to convert "partition" to a mutable pointer.

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

playlist_file_print() already handles stored playlists.  spl_print()
is never called with a successful result.

Wrapper for protocol/ArgParser.cxx.