__uacme_obtain: allow use of stdin with the --renew-hook parameter

type/__uacme_obtain/man.rst

@@ -38,7 +38,8 @@ install-key-to
   Installation path of the certificate's private key.
 
 renew-hook
-  Renew hook executed on certificate renewal (e.g. `service nginx reload`).
+  Renew hook executed on certificate renewal (e.g. `service nginx reload`, `-`
+  for the standard input).
 
 force-cert-ownership-to
   Override default ownership for TLS certificate, passed as argument to chown.

type/__uacme_obtain/manifest

@@ -109,7 +109,11 @@ export CERT_TARGET
 RENEW_HOOK=
 if [ -f "${__object:?}/parameter/renew-hook" ];
 then
-	RENEW_HOOK="$(cat "${__object:?}/parameter/renew-hook")"
+	if [ "$(cat "${__object:?}/parameter/renew-hook")" = "-" ]; then
+		RENEW_HOOK="$(cat ${__object:?}/stdin)"
+	else
+		RENEW_HOOK="$(cat "${__object:?}/parameter/renew-hook")"
+	fi
 fi
 export RENEW_HOOK