Fixes Travis failure with Meson 0.50:

 ERROR: Could not detect Ninja v1.5 or newer

Split several printf() calls to make it easier to deal with all those
#ifdefs.

Copy the system socket unit to the "user" directory.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/530

This allows using the file as a user unit, where "%t" maps to
"$XDG_RUNTIME_DIR".

Proposed in https://github.com/MusicPlayerDaemon/MPD/issues/530

Closes https://github.com/MusicPlayerDaemon/MPD/issues/524

Closes https://github.com/MusicPlayerDaemon/MPD/issues/523

Expect OnSocketReady() to cancel events.  If it returns false, the
SocketMonitor may be destructed already.  This fixes a use-after-free
bug in the "httpd" output plugin.

Keep the SocketMonitor registered.  This wrong return value was added
6 years ago in commit 72cf8dd8, andd
apparently, nobody ever noticed.

This missing piece probably never really hurt, because
HttpdClient::OnSocketClosed() would be called right after a socket
error, but it's better to be explicit about closing on error.

Fixes:

 src/net/IPv4Address.hxx: In member function 'constexpr IPv4Address::operator SocketAddress() const':
 src/net/IPv4Address.hxx:171:24: error: a reinterpret_cast is not a constant expression
   171 |   return SocketAddress((const struct sockaddr *)&address,
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 src/net/IPv6Address.hxx: In member function 'constexpr IPv6Address::operator SocketAddress() const':
 src/net/IPv6Address.hxx:138:24: error: a reinterpret_cast is not a constant expression
   138 |   return SocketAddress((const struct sockaddr *)&address,
       |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Closes https://github.com/MusicPlayerDaemon/MPD/issues/522

Let the caller decide what to do with the original exception.

Preserve the original exception.

This plugin is interesting only for a tiny fraction of MPD users, so
let's not spam everybody else's log with it.

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

Fixes #184.

Semaphores are kernel-managed objects, calling delete_sem() twice is not more
dangerous than calling close() twice on an fd though, it would just return
an error.

The custom_command was run in src/haiku/ and created a file with only resources inside.

Since xres edits the file in-place and meson doesn't like it, we have to run a shell script for now.
Maybe later I'll add proper support in meson.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/510

.. and not "UNIX domain socket.  Be consistent about the naming.