Generate `/etc/iproute2/{rt_protos.d,rt_tables.d}/nextmn.conf` with a command

[louisroyer]

• hidden command, can be used to build with Docker easily (instead of doing this: https://github.com/louisroyer-docker/nextmn/blob/8f1eab7faab5af8481dc9a7f1cbd403b3471616e/srv6/Dockerfile#L28-L29)
• add it to Makefile

$ srv6 iproute2 rt_tables > /etc/iproute2/rt_tables.d/nextmn.conf
$ srv6 iproute2 rt_protos > /etc/iproute2/rt_protos.d/nextmn.conf

[louisroyer]

in Makefile, install to /etc/iproute2 when prefix is /usr/local, else install to $prefix/iproute2