mount: convert to VERB_COMMAND and add --introspect-cli

The custom help() is replaced by a COMMAND() description, and
--introspect-cli is added.

Options are now in two groups: "Common options" and Mount options". In
--help, for systemd-mount, both groups are shown. For systemd-umount, only
the first. This changes the behaviour a bit: previously, both groups were
shown together as one list. I think the new behaviour is actually a bit
better, because it makes it easier for the user to figure out that the
commands share common options and one of the commands has its private
second group. But if people think that the previous behaviour should
be restored, we could add a display options to squish the groups back
together.

--help now goes through the pager, like other commands with --no-pager.

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

@@ -80,6 +80,7 @@ INTROSPECTABLE=(
     systemd-machine-id-setup
     systemd-measure
     systemd-modules-load
+    systemd-mount
     systemd-mstack
     systemd-mute-console
     systemd-network-generator
@@ -119,6 +120,7 @@ INTROSPECTABLE=(
     systemd-tpm2-clear
     systemd-tpm2-setup
     systemd-tty-ask-password-agent
+    systemd-umount
     systemd-update-done
     systemd-validatefs
     systemd-veritysetup
@@ -181,6 +183,15 @@ systemd-dissect --introspect-cli | jq -e \
 systemd-id128 --introspect-cli | jq -e \
     '.commands[0].verbs | map(.names[0]) | contains(["new", "machine-id", "show", "help"])'
 
+if command -v systemd-mount >/dev/null; then
+    systemd-mount --introspect-cli | jq -e \
+        '[.commands[].names[0]] | sort == ["systemd-mount", "systemd-umount"]'
+
+    # --tmpfs is only valid for systemd-mount
+    (! systemd-mount --tmpfs 2>&1) | grep "one argument required" >/dev/null
+    (! systemd-umount --tmpfs 2>&1) | grep "unrecognized option" >/dev/null
+fi
+
 systemd-mstack --introspect-cli | jq -e \
     '[.commands[].names[0]] | sort == ["mount.mstack", "systemd-mstack"]'