nixos/activation: Rename "tmpfs" to "specialfs"

Using "tmpfs" as a script part for system.activationScripts is a bit
misleading since 6efcfe03ae.

We no longer solely mount tmpfs within this script, so using "specialfs"
fits more nicely in terms of naming.

Tested against the "simple" NixOS installer test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

nixos/modules/config/networking.nix

@@ -232,7 +232,7 @@ in
     # and other configurations. If the file is destroyed by an environment
     # activation then it must be rebuilt so that applications which interface
     # with /etc/resolv.conf directly don't break.
-    system.activationScripts.resolvconf = stringAfter [ "etc" "tmpfs" "var" ]
+    system.activationScripts.resolvconf = stringAfter [ "etc" "specialfs" "var" ]
       ''
         # Systemd resolved controls its own resolv.conf
         rm -f /run/resolvconf/interfaces/systemd

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

@@ -159,7 +159,7 @@ in
         rmdir --ignore-fail-on-non-empty /usr/bin /usr
       '';
 
-    system.activationScripts.tmpfs =
+    system.activationScripts.specialfs =
       ''
         specialMount() {
           local device="$1"