Move primary interface sysctl config to netdog

[zmrow]

Description of changes:

    systemd: rename var-run-tmpfiles to systemd-tmpfiles
    
    This renames var-run-tmpfiles to systemd-tmpfiles to better match
    convention in our other packages

    systemd: Add /etc/sysctl.d to tmpfiles
    
    This adds a line to `systemd-tmpfiles` to ensure that the
    `/etc/sysctl.d` directory is created at boot.

    Move primary interface sysctl config to netdog
    
    eth0 is not the default interface for all variants any longer.  This
    change removes the hardcoded `eth0` specific sysctls from default
    settings.  `netdog` is now responsible for setting these default sysctls
    for whatever the primary interface is for the platform.
    
    In order to do this, a new subcommand `prepare-primary-interface` is
    added to `netdog` and run via a oneshot service.  The command reads the
    primary interace from file (previously generated by `netdog`) and writes
    the sysctls for that interface to file in `/etc/sysctl.d`.  It then
    executes `systemd-sysctl`, which sets these sysctls.  Since all of this
    happens early in boot, a user is able to override these via API settings
    as usual.

Testing done:
Boot aws-k8s-1.19, aws-k8s-1.22, and metal-dev and ensure the sysctl file is written and sysctls are set:

bash-5.1# cat /etc/sysctl.d/90-primary_interface.conf 
net.ipv6.conf.eth0.accept_ra = 2
net.ipv4.conf.eth0.rp_filter = 2

bash-5.1# sysctl net.ipv6.conf.eth0.accept_ra
net.ipv6.conf.eth0.accept_ra = 2

bash-5.1# sysctl net.ipv4.conf.eth0.rp_filter
net.ipv4.conf.eth0.rp_filter = 2

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[bcressey]

What takes care of applying the equivalent of this change to the primary interface on metal hosts? It feels kind of rough to expect the admin to discover they also need to add these sysctl settings.

Since we have the concept of primary interface in netdog now, I'd prefer letting netdog own this via a new oneshot service that runs at the right point in the boot sequence: after /proc/sys/net/ipv4/conf/<primary> exists; before settings are applied so that sysctl settings can still be used to override the defaults.

[zmrow]

^ Fixes per @bcressey 's suggestions