Add an option to the UPnP database plugin to configure which interface
is used by upnp to discover servers.
upnp by default selects the first interface that is not loopback, which
in some cases might not be the desired interface. For example if wanting
to access a DLNA server over a VPN connection.
The "interface" option can now be set to the name of the desired
interface to achieve this.
The default behaviour remains unchanged.
Adds the Interface Name as an argument to the *Init functions to make it
possible to select which interface is used by upnp to detect servers.
Currently "nullptr" is passed in to let the upnp library select an
interface, as before.
Commit 552c30eae caused problems for those people who still had a
"pid_file" setting (even though that is obsolete with systemd),
because now /run/mpd is owned by root:root (our mpd.service has no
User=mpd directive, so systemd starts MPD as root).
To work around this problem, and to be able to keep
RuntimeDirectory=mpd (which solved a problem of other MPD users), the
best compromise seems to just ignore the "pid_file" setting when it is
of no use.
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.
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
