Add support for DragonFly BSD

etc/rc.conf.DragonFly

@@ -0,0 +1,13 @@
+##############################################################################
+# DragonFly BSD SPECIFIC OPTIONS
+
+# This is the subsystem type. Valid options on DragonFly BSD:
+# ""        - nothing special
+# "jail"    - DragonFly BSD jails
+# "prefix"  - Prefix
+# If this is commented out, automatic detection will be used.
+#
+# This should be set to the value representing the environment this file is
+# PRESENTLY in, not the virtualization the environment is capable of.
+#rc_sys=""
+