network: add support for CIDR addresses Debian routes

OnMetal is changing their template from:
`route add -net 1.2.3.0 netmask 255.255.255.0 gw 10.1.2.1 || true`
to:
`route add -net 1.2.3.0/24 gw 10.1.2.1 || true`
That's my inline implementation of Must ParseCIDR().