Fixes a build failure in `CommandLine.cxx`.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/905

release v0.21.24

Our Windows build is built with `--disable-netrc`, and that makes
CURLOPT_NETRC fail, causing failures with all streams.  D'oh!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/886

Closes https://github.com/MusicPlayerDaemon/MPD/issues/891

Closes https://github.com/MusicPlayerDaemon/MPD/issues/893

When the client wants to seek, but the decoder has already finished
decoding the current song, the player restarts the decoder with an
initial seek at the new position.  When this initial seek fails, MPD
pretends nothing has happened and plays this song from the start.

With this new flag, a restarted decoder marks the initial seek as
"essential" and fails the decoder if that seek fails.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/895

The io_uring buffer is allocated as memlocked memory, as MPD needs to
be able to lock memory.

Before the advent of io_uring (commit dae8da70), this didn't
matter, because the `FileInputStream` never called this.  But
`UringInputStream` is derived from `AsyncInputStream`, and needs the
handler to signal completion.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/898

For subprojects.

c++17 is offered by default with Xcode 10
https://developer.apple.com/documentation/xcode_release_notes/xcode_10_release_notes

The ones in std have overloads for const char/char.
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Passing `length+1` to `MultiByteToWideChar()` means the function may
fill the whole buffer with output data, and could theoretically
overwrite the null terminator.  In practice, this will never happen,
but this way, it's slightly more correct.

Also, null-terminate after `MultiByteToWideChar()`, after we got the
real output length.  Again, this would never have been a problem, but
who knows...

Needed for std::wmemchr under libcxx
Signed-off-by: Rosen Penev <rosenp@gmail.com>