cmd/nginx-auth: maintainer scripts and tailnet checking (#4460)

* cmd/nginx-auth: add maintainer scripts

Signed-off-by: Xe <xe@tailscale.com>

* cmd/nginx-auth: add Expected-Tailnet header and documentation

Signed-off-by: Xe <xe@tailscale.com>

cmd/nginx-auth/rpm/postrm.sh

@@ -0,0 +1,9 @@
+# $1 == 0 for uninstallation.
+# $1 == 1 for removing old package during upgrade.
+
+systemctl daemon-reload >/dev/null 2>&1 || :
+if [ $1 -ge 1 ] ; then
+    # Package upgrade, not uninstall
+    systemctl stop tailscale.nginx-auth.service >/dev/null 2>&1 || :
+    systemctl try-restart tailscale.nginx-auth.socket >/dev/null 2>&1 || :
+fi

cmd/nginx-auth/rpm/prerm.sh

@@ -0,0 +1,9 @@
+# $1 == 0 for uninstallation.
+# $1 == 1 for removing old package during upgrade.
+
+if [ $1 -eq 0 ] ; then
+    # Package removal, not upgrade
+    systemctl --no-reload disable tailscale.nginx-auth.socket > /dev/null 2>&1 || :
+    systemctl stop tailscale.nginx-auth.socket > /dev/null 2>&1 || :
+    systemctl stop tailscale.nginx-auth.service > /dev/null 2>&1 || :
+fi