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

There is already one call in ReadServers(), which is the correct place
to do it.

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.