diod: drop program prefix from logs

Problem: the "diod: " log prefix is not helpful because server
logging is rarely mixed with output from other programs.

Or if it is, it's been processed through the systemd journal or
syslog which also adds its own prefix.

Initialize server logging with no prefix.

src/cmd/diod.c

@@ -121,7 +121,7 @@ main(int argc, char **argv)
     srvmode_t mode = SRV_NORMAL;
     int rfdno = -1, wfdno = -1;
 
-    diod_log_init (argv[0]);
+    diod_log_init (NULL);
     diod_conf_init ();
 
     /* config file overrides defaults */