add prefixlength option to "ip rule"

When configuring a system with multiple network uplinks and default routes, it
is often convenient to reference the main routing table multiple times - but
omitting the default route. Using this modified "ip" utility and the
corresponding kernel patch, this can be achieved by using the following command
sequence:

$ ip route add table secuplink default via 10.42.23.1

$ ip rule add pref 100            table main prefixlength 1
$ ip rule add pref 150 fwmark 0xA table secuplink

With this setup, packets marked 0xA will be processed by the additional routing
table "secuplink", but only if no suitable route in the main routing table can
be found. By using a minimum prefixlength of 1, the default route (/0) of the
table "main" is hidden to packets processed by rule 100; packets traveling to
destinations via more specific routes are processed as usual.