Add enable_pvp setting

minetest.conf.example

@@ -133,6 +133,8 @@
 #default_privs = build, shout
 # Whether players are transferred to client without any range limit
 #unlimited_player_transfer_distance = true
+# Whether to enable players killing each other
+#enable_pvp = true
 
 # Profiler data print interval. #0 = disable.
 #profiler_print_interval = 0