nixos/pam: create wtmp/lastlog iff using pam_lastlog

I think pam_lastlog is the only thing that writes to these files in
practice on a modern Linux system, so in a configuration that doesn't
use that module, we don't need to create these files.

I used tmpfiles.d instead of activation snippets to create the logs.
It's good enough for upstream and other distros; it's probably good
enough for us.

nixos/modules/system/activation/activation-script.nix

@@ -128,11 +128,6 @@ in
       ''
         # Various log/runtime directories.
 
-        mkdir -m 0755 -p /var/log
-
-        touch /var/log/wtmp /var/log/lastlog # must exist
-        chmod 644 /var/log/wtmp /var/log/lastlog
-
         mkdir -m 1777 -p /var/tmp
 
         # Empty, immutable home directory of many system accounts.