bugfix: Correctly use QProcess::start and QProcess::execute (#1331)

Affected functions (all on Linux/Mac):
 - `RouterLinux::flushDns` was not reloading the DNS manager.
 - `Utils::processIsRunning` was always saying that the process
   is not running when `fullFlag` was set to `false`.
 - `Utils::killProcessByName` was not killing anything.

service/server/router_linux.cpp

@@ -162,11 +162,11 @@ void RouterLinux::flushDns()
         || QFileInfo::exists("/usr/sbin/nscd")
         || QFileInfo::exists("/usr/lib/systemd/system/nscd.service"))
     {
-        p.start("systemctl restart nscd");
+        p.start("systemctl", { "restart", "nscd" });
     }
     else
     {
-        p.start("systemctl restart systemd-resolved");
+        p.start("systemctl", { "restart", "systemd-resolved" });
     }
 
     p.waitForFinished();