org.osbuild.systemd: Support masking generators

This adds a new key masked_generators, similar to masked_services,
which masks systemd generators from running at boot, by creating
symlinks to /dev/null in /etc/systemd/systemd-generators, as
described in:
 https://www.freedesktop.org/software/systemd/man/latest/systemd.generator.html#Description

This will be useful for the automotive project, as it allows disabling
of unsupported things like sysv or rc.local legacy support, while
improving boot performance.

test/data/stages/systemd/b.json

@@ -649,6 +649,9 @@
             "masked_services": [
               "ldconfig"
             ],
+            "masked_generators": [
+              "systemd-sysv-generator"
+            ],
             "disabled_services": [
               "sshd"
             ]

test/data/stages/systemd/b.mpp.yaml

@@ -35,5 +35,7 @@ pipelines:
             - nftables
           masked_services:
             - ldconfig
+          masked_generators:
+            - systemd-sysv-generator
           disabled_services:
             - sshd

test/data/stages/systemd/diff.json

@@ -1,7 +1,9 @@
 {
   "added_files": [
     "/etc/systemd/system/ldconfig.service",
-    "/etc/systemd/system/multi-user.target.wants/nftables.service"
+    "/etc/systemd/system/multi-user.target.wants/nftables.service",
+    "/etc/systemd/system-generators",
+    "/etc/systemd/system-generators/systemd-sysv-generator"
   ],
   "deleted_files": [
     "/etc/systemd/system/multi-user.target.wants/sshd.service"