Change unbound logfile to the empty string

This is defined to send log messages to stderr, which is
what we want - fixes #1536 ("Could not open logfile /dev/stdout:
Permission denied")

optional/unbound/unbound.conf

@@ -2,7 +2,7 @@ server:
   verbosity: 1
   interface: 0.0.0.0
   interface: ::0
-  logfile: /dev/stdout
+  logfile: ""
   do-ip4: yes
   do-ip6: yes
   do-udp: yes