resolve shellcheck offenses

With nsupdate the rule seems to be: filenames need to be wrapped
in double quotes, while all other options must not use double quotes.
Hence there is no way to resolve the shellcheck offense, because
the key requires quotes, but the other options must not use quotes.

dnsapi/dns_nsupdate.sh

@@ -39,12 +39,14 @@ dns_nsupdate_add() {
   [ -n "$DEBUG" ] && [ "$DEBUG" -ge "$DEBUG_LEVEL_1" ] && nsdebug="-d"
   [ -n "$DEBUG" ] && [ "$DEBUG" -ge "$DEBUG_LEVEL_2" ] && nsdebug="-D"
   if [ -z "${NSUPDATE_ZONE}" ]; then
+    #shellcheck disable=SC2086
     nsupdate -k "${NSUPDATE_KEY}" $nsdebug $NSUPDATE_OPT <<EOF
 server ${NSUPDATE_SERVER}  ${NSUPDATE_SERVER_PORT}
 update add ${fulldomain}. 60 in txt "${txtvalue}"
 send
 EOF
   else
+    #shellcheck disable=SC2086
     nsupdate -k "${NSUPDATE_KEY}" $nsdebug $NSUPDATE_OPT <<EOF
 server ${NSUPDATE_SERVER}  ${NSUPDATE_SERVER_PORT}
 zone ${NSUPDATE_ZONE}.
@@ -77,12 +79,14 @@ dns_nsupdate_rm() {
   [ -n "$DEBUG" ] && [ "$DEBUG" -ge "$DEBUG_LEVEL_1" ] && nsdebug="-d"
   [ -n "$DEBUG" ] && [ "$DEBUG" -ge "$DEBUG_LEVEL_2" ] && nsdebug="-D"
   if [ -z "${NSUPDATE_ZONE}" ]; then
+    #shellcheck disable=SC2086
     nsupdate -k "${NSUPDATE_KEY}" $nsdebug $NSUPDATE_OPT <<EOF
 server ${NSUPDATE_SERVER}  ${NSUPDATE_SERVER_PORT}
 update delete ${fulldomain}. txt
 send
 EOF
   else
+    #shellcheck disable=SC2086
     nsupdate -k "${NSUPDATE_KEY}" $nsdebug $NSUPDATE_OPT <<EOF
 server ${NSUPDATE_SERVER}  ${NSUPDATE_SERVER_PORT}
 zone ${NSUPDATE_ZONE}.