[klibc] ipconfig: Implement classless static routes

Implement classless static routes support as specified in RFC3442.

Bug-Debian: https://bugs.debian.org/884716
Bug-Ubuntu: https://launchpad.net/bugs/1526956
Signed-off-by: Benjamin Drung <benjamin.drung@profitbricks.com>
Link: https://salsa.debian.org/kernel-team/klibc/merge_requests/2
Link: https://www.zytor.com/pipermail/klibc/2018-June/003993.html
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>