start-stop-daemon: Add SSD_IONICELEVEL

This is the disk IO counterpart to SSD_NICELEVEL.
Modified by William Hubbs to add the variable to the start-stop-daemon
man page.

This fixes #69.

etc/rc.conf

@@ -117,6 +117,9 @@
 # Some daemons are started and stopped via start-stop-daemon.
 # We can set some things on a per service basis, like the nicelevel.
 #SSD_NICELEVEL="-19"
+# Or the ionice level. The format is class[:data] , just like the
+# --ionice start-stop-daemon parameter.
+#SSD_IONICELEVEL="2:2"
 
 # Pass ulimit parameters
 # If you are using bash in POSIX mode for your shell, note that the