Due to the automatic inclusion of DEFAULTS everywhere, and since it

has device mem in it almost everywhere, we get warnings about
duplicated device almost everywhere.  Comment it out, with a note
about why, so that we don't get those warnings.

sys/conf/NOTES

@@ -1110,7 +1110,8 @@ options 	VFS_AIO
 device		random
 
 # The system memory devices; /dev/mem, /dev/kmem
-device		mem
+# In various system's DEFAULTS file, so we don't need it here.
+#device		mem
 
 # The kernel symbol table device; /dev/ksyms
 device		ksyms