This is the final piece of the series to establish io_uring support on
Linux.

MPD doesn't need io_uring for its efficient bulk I/O support, but to
allow file I/O to be cancelled.  This is a big problem on CIFS/NFS
mounts where processes sleep uninterruptable if the file server
disappears, deadlocking MPD.

With io_uring, a flaky NFS connection allows MPD to continue to work
(even though there are still deadlocks inside MPD which need to be
addressed).

This plugin does not yet use cancellable `open()` using
`IORING_OP_OPENAT`.  This will be implemented later.

Lots of other optimization opportunities for io_uring are still
missing as well - for example the database update could benefit a lot,
but unfortunately, io_uring doesn't have `readdir()` support just yet.

Relax the assertions.  This is necessary if BlockingCall() is used
before the thread is started.

This makes ccache more efficient when recompiling with different
plugins.

The parser implemented in libmpdclient requires the first key-value pair
of the server response to be the file pair. This is due to the fact that
libmpdclient scan pairs sequentially and first attempts to extract the
file pair before parsing the currentsong response further. See:

	https://github.com/MusicPlayerDaemon/libmpdclient/blob/5c751a761ec9f71a43ee0e41cfff3208f31a58b8/src/song.c#L559-L563

Meta data encoded as pairs in the currentsong response will be ignored
if they are placed before the file pair in the response.

Fixes breakage of non-ASCII characters, regression from commit
cc72ceb3

Fixes https://github.com/MusicPlayerDaemon/MPD/issues/842

Prepare to fix cc72ceb3

Also include fade-out time in song length.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/838

This reduces resource waste for resizing the std::vector in most
cases.

Optimizes a few nanoseconds from the common code path.

release v0.21.23

libnfs is compiled with `-D_FILE_OFFSET_BITS=64`, but Meson decides
not to enable this mode.  We could force this mode, but then again,
these days, nobody should be using 32-bit Windows ... so this is a
kludge only for debugging with 32-bit WINE.

Fixes Path::IsNull() checks on Windows.

This fixes a freeze bug in the NFS input/storage plugins: when libnfs
auto-reconnets after a failure, it installs the new socket on the same
file descriptor number.  MPD's attempt to unregister the old socket by
calling SocketMonitor::Steal() from NfsConnection::ScheduleSocket()
fails because the new/old socket number is not registered in epoll, so
epoll_ctl() returns ENOENT.  The problem is that it left
`scheduled_flags`, and so subsequent Schedule() calls will use
`EPOLL_CTL_MOD`, which will fail again and again.  Instead, we need to
use `EPOLL_CTL_ADD` to register the new socket.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/806

Closes https://github.com/MusicPlayerDaemon/MPD/issues/756

This flag is output-only.

These flags are output-only.

These flags are output-only.  Using them here is misleading.

The object's state is `IDLE` when OnNfsCallback() gets invoked, so
let's use the start of the method to reset the `state` field.

Fixes assertion failure if the callback fails.