stages/systemd: ability to mask services

Add the ability to mask services, which is done e.g. when building
installers. See systemctl(1) for more information about masked
services.
Modify the existing stage test to include a test for it.

test/data/stages/systemd/b.json

@@ -826,6 +826,9 @@
           "enabled_services": [
             "nftables"
           ],
+          "masked_services": [
+            "ldconfig"
+          ],
           "disabled_services": [
             "sshd"
           ]

test/data/stages/systemd/diff.json

@@ -1,5 +1,6 @@
 {
   "added_files": [
+    "/etc/systemd/system/ldconfig.service",
     "/etc/systemd/system/multi-user.target.wants/nftables.service"
   ],
   "deleted_files": [