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

Without clearing the "uri" field, the next Open() call attempts to
reuse the old InputStream, but it has already been closed, so Open()
always returns nullptr.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1300

Almost all callers have string literal, and the length is known at
compile time.

The inverted check was introduced by commit 46d00dd8, and commit
8ad17d25 added a check for the wrong variable.  D'oh!

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1303