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

Closes https://github.com/MusicPlayerDaemon/MPD/issues/516

Disable the 0.11 compatibility mode if the only argument is a filter
expression.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/506

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.

Must lock the mutex before calling any of the unprotected InputStream methods.

Closes #462

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.

Closes #389

Grouping in the "list" command was completely broken from the start,
unlike "count group".  I have no idea what I have been thinking when I
wrote commit ae178c77, but it didn't
make any sense.

This commit is a rewrite of the feature.

For clients to be able to detect this feature, this commit also
increments the protocol version.

Keep MPD's footprint small until playback is requested.

Remove them as parameters from `db_selection_print()`.

Fixes `-Werror=odr`

Prepare to allow more complex expressions.

Will allow more complex fitler expression, such as negation (#89).