Show error if chain is configured for a CA which doesn't offer alternate chains (fixes #845)

dehydrated

@@ -1198,6 +1198,9 @@ sign_csr() {
     crt="$(signed_request "${certificate}" "" 4>"${resheaders}")"
 
     if [ -n "${PREFERRED_CHAIN:-}" ]; then
+      if ! (grep -Ei '^link:' "${resheaders}" | grep -q -Ei 'rel="alternate"'); then
+        _exiterr "Preferred chain defined but CA doesn't offer chain selection."
+      fi
       foundaltchain=0
       altcn="$(get_last_cn "${crt}")"
       altoptions="${altcn}"