Prolong wait time for shutdown

The default was 90s[1], prolong to 30 mins.
From the discussion in [2], ~2k torrents took 5 mins to completely
shutdown. Here we wait at most 30 mins which scales to about 12k
torrents which should cover most use case (also note that 4.3.x series
is mentioned to have even shorter shutdown time).

[1] https://www.freedesktop.org/software/systemd/man/systemd-system.conf.html#DefaultTimeoutStartSec=
[2] https://github.com/qbittorrent/qBittorrent/issues/15381#issuecomment-912080617

PR #15414.

dist/unix/systemd/qbittorrent-nox@.service.in

@@ -9,6 +9,7 @@ Type=simple
 PrivateTmp=false
 User=%i
 ExecStart=@EXPAND_BINDIR@/qbittorrent-nox
+TimeoutStopSec=1800
 
 [Install]
 WantedBy=multi-user.target