nixos/systemd: install sysctl snippets

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/modules/system/boot/systemd.nix

@@ -831,6 +831,10 @@ in
         [Sleep]
       '';
 
+      # install provided sysctl snippets
+      "sysctl.d/50-coredump.conf".source = "${systemd}/example/sysctl.d/50-coredump.conf";
+      "sysctl.d/50-default.conf".source = "${systemd}/example/sysctl.d/50-default.conf";
+
       "tmpfiles.d/systemd.conf".source = "${systemd}/example/tmpfiles.d/systemd.conf";
       "tmpfiles.d/x11.conf".source = "${systemd}/example/tmpfiles.d/x11.conf";