log: add syslog support

Add support to log using standard syslog,
add related option in dhtnode.
dhtnode will use syslog by default
when starting in daemon mode and
no logfile is defined.

tools/dhtnode.cpp

@@ -370,7 +370,9 @@ main(int argc, char **argv)
         dht->run(params.port, crt, true, params.network);
 
         if (params.log) {
-            if (not params.logfile.empty())
+            if (params.syslog or (params.daemonize and params.logfile.empty()))
+                log::enableSyslog(*dht, "dhtnode");
+            else if (not params.logfile.empty())
                 log::enableFileLogging(*dht, params.logfile);
             else
                 log::enableLogging(*dht);