Stop ipfw from aborting when asked to delete a table entry that

Stop ipfw from aborting when asked to delete a table entry that
doesn't exist or add one that is already present, if the -q flag
is set. Useful for "ipfw -q /dev/stdin" when the command above is
invoked from  something like python or TCL to feed commands
down the throat of ipfw.
MFC in: 1 week