fix(sshinstall.sh): support non-systemd init (SysVinit, e.g. MX Linux)

The service step called `systemctl enable --now`, which aborts under
`set -e` when systemd is not PID 1. On MX Linux (SysVinit) this meant sshd
was never started and the firewall and admin-key steps were never reached.

Detect systemd via /run/systemd/system; otherwise fall back to
`update-rc.d`/`chkconfig` + `service <name> start`, failing with a clear
message if the service cannot be started.

Found on the first real-machine run (ThinkCentre M71e, MX Linux 25.2).

Co-Authored-By: Claude Sonnet 5 <[email protected]>

sshinstall.sh

@@ -88,12 +88,31 @@ esac
 
 # ── Enable + start service ────────────────────────────────────────
 header "SSH service"
-if systemctl is-active --quiet "$SERVICE"; then
-    skip "$SERVICE service"
+if [[ -d /run/systemd/system ]]; then
+    # systemd is PID 1
+    if systemctl is-active --quiet "$SERVICE"; then
+        skip "$SERVICE service"
+    else
+        info "Enabling + starting $SERVICE..."
+        systemctl enable --now "$SERVICE"
+        success "$SERVICE running"
+    fi
 else
-    info "Enabling + starting $SERVICE..."
-    systemctl enable --now "$SERVICE"
-    success "$SERVICE running"
+    # SysVinit / no systemd (e.g. MX Linux default) — use the classic tools
+    warn "systemd is not running — using SysV init scripts"
+    if service "$SERVICE" status &>/dev/null; then
+        skip "$SERVICE service"
+    else
+        info "Enabling + starting $SERVICE (SysV)..."
+        if command -v update-rc.d &>/dev/null; then
+            update-rc.d "$SERVICE" defaults &>/dev/null || true
+            update-rc.d "$SERVICE" enable &>/dev/null || true
+        elif command -v chkconfig &>/dev/null; then
+            chkconfig "$SERVICE" on || true
+        fi
+        service "$SERVICE" start || die "Could not start $SERVICE via 'service' — check /etc/init.d/$SERVICE"
+        success "$SERVICE running"
+    fi
 fi
 
 # ── Firewall ───────────────────────────────────────────────────────