ifupdown: 'prio' should have been 'metric' in static_up and static_up6

ifupdown: 'prio' should have been 'metric' in static_up and static_up6

When a static interface with explicit gateway and metric in /etc/network/interfaces
is enabled using 'ifup', and further, the BusyBox 'ip' applet is enabled, the
following error message appears at the console (and no default route is added):

ip: either "to" is duplicate, or "prio" is garbage

Tracing ifup reveals that it is attempting to run the following shell command:

ip route add default via <GW> dev <DEVICE> prio <METRIC>

'ip' does not understand the 'prio' argument, causing this error. With 'metric',
it works fine.

Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>