pppd: Restore adding a default route with different metric (#472)

For instance, when using a ppp link as backup link, one would want to
add a default route on the ppp link, in *addition* to the existing
default route.

d0ccb87156c2 ("pppd: Add replacedefaultroute option (#200)") however broke
this case: sifdefaultroute was not passing the metric to
defaultroute_exists any more. This commit restores this case.

Fixes #357

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>