This way, MPD can reliably detect whether it was started as systemd
service, which is better than checking sd_booted(), which only checks
whether systemd manages all services, but still MPD could be started
manually.

Signed-off-by: Shen-Ta Hsieh <ibmibmibm.tw@gmail.com>

Signed-off-by: Shen-Ta Hsieh <ibmibmibm.tw@gmail.com>

Fixes assertion failure when the EventLoop gets destructed before
Run() was ever called.

Fixes https://bugs.debian.org/998310

With the "wave" encoder, this has no effect, but it's more correct.

This uncomments the code which had been present already in the first
Snapcast commit (copied from the "httpd" output plugin), but I
commented it because I did not know whether I needed to send silence
samples to all Snapcast clients.

As a side effect, this fixes playback when no Snapcast client is
connected; this was broken because Pause() always returned a positive
value when there were no clients.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1310

Unlike GetFilenameSuffix(), uri_get_suffix() removes the query string
first, which breaks file names with question marks in the name.
Therefore, uri_get_suffix() shall only be applied to remote URIs.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1316

Regression by commit 2d8847f4

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1106

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1285