sysctl.d(5) recommends prefixing all filenames in /etc/sysctl.d with a
two-digit number and a dash, to simplify the ordering of the files.

Some packages provide custom files, often with "50-" prefix.
To ensure user-supplied configuration takes precedence over the one
specified via `boot.kernel.sysctl`, prefix the file generated there with
"60-".

systemd provides two sysctl snippets, 50-coredump.conf and
50-default.conf.

These enable:
 - Loose reverse path filtering
 - Source route filtering
 - `fq_codel` as a packet scheduler (this helps to fight bufferbloat)

This also configures the kernel to pass coredumps to `systemd-coredump`.
These sysctl snippets can be found in `/etc/sysctl.d/50-*.conf`,
and overridden via `boot.kernel.sysctl`
(which will place the parameters in `/etc/sysctl.d/60-nixos.conf`.

Let's start using these, like other distros already do for quite some
time, and remove those duplicate `boot.kernel.sysctl` options we
previously did set.

In the case of rp_filter (which systemd would set to 2 (loose)), make
our overrides to "1" more explicit.

nixos/systemd: install sysctl snippets