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/config/sysctl.nix

@@ -52,12 +52,6 @@ in
         restartTriggers = [ config.environment.etc."sysctl.d/60-nixos.conf".source ];
       };
 
-    # Enable hardlink and symlink restrictions.  See
-    # https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=800179c9b8a1e796e441674776d11cd4c05d61d7
-    # for details.
-    boot.kernel.sysctl."fs.protected_hardlinks" = true;
-    boot.kernel.sysctl."fs.protected_symlinks" = true;
-
     # Hide kernel pointers (e.g. in /proc/modules) for unprivileged
     # users as these make it easier to exploit kernel vulnerabilities.
     boot.kernel.sysctl."kernel.kptr_restrict" = 1;