Fix error produced by pfctl when no rules are returned

[tschettervictor]

When adding a rdr rule, pfctl will error when it does not find any rules using pfctl -a rdr/jail -Psn;

Elsewhere in the script, this is solved by simply adding 2>/dev/null to not return errors. This PR simply adds the same functionality to the "load_rdr_rule" function.

[tschettervictor]

#673 #644

This one took a while to solve, but I finally discovered that the error only happens when pfctl cannot find any rules using the Psn flags.

[bmac2]

approve