add static routes prio to default gateway

Some hosters use a /32 allocation to save IP addresses and secure their
network. These addresses can either be configured as point-to-point
(a.k.a. peer in iproute2) which netctl does not support, or by injecting
a static route to the default gateway, e.g.:

ip addr add 1.2.3.4/32 dev eth0
ip route add 1.2.3.1 dev eth0
ip route add default via 1.2.3.1

However this does not work with netctl because the default gateway is
set prior to other static routes.

This patch fixes this issue.