dissect: describe the mount.ddi helper interface

Add a second COMMAND() for the mount.ddi helper namespace, like for
mount.storage and mount.mstack, so that it is exposed by --introspect-cli
too.

Fixes 1a54ce154c +
66246c98b5.

src/dissect/dissect.c

@@ -712,6 +712,13 @@ static int parse_argv(int argc, char *argv[]) {
         return 1;
 }
 
+COMMAND(
+        "mount.ddi\0",
+        "External helper for mount.8 to mount Discoverable Disk Images (DDIs).",
+        .man_pages = "systemd-dissect.1\0",
+        .option_namespace = "mount.ddi",
+);
+
 static int parse_argv_as_mount_helper(int argc, char *argv[]) {
         const char *options = NULL;
         bool fake = false;

test/units/TEST-74-AUX-UTILS.introspect-cli.sh

@@ -140,6 +140,9 @@ storagectl --introspect-cli | jq -e \
 systemd-clonesetup --introspect-cli | jq -e \
     '.commands[0].verbs | map(.names[0]) | sort == ["add", "remove"]'
 
+systemd-dissect --introspect-cli | jq -e \
+    '[.commands[].names[0]] | sort == ["mount.ddi", "systemd-dissect"]'
+
 systemd-id128 --introspect-cli | jq -e \
     '.commands[0].verbs | map(.names[0]) | contains(["new", "machine-id", "show", "help"])'