Add dns_he_dyntxt, a simpler dns.he.net API.

These are the pros and cons of dns_he_dyntxt, compared to dns_he:
Pros:
- No need to store a dns.he.net account password on your server
- Uses a very simple write-only API
Cons:
- You must manually create placeholder _acme-challenge TXT records,
  and generate/copy the same DDNS key across all records.
- This script WILL FAIL to issue both a domain and its wildcard, because
  '-d example.com -d *.example.com' requires multiple TXT records.
  Switch to 'dns_he' if you need this feature.

dnsapi/dns_he_dyntxt.sh

@@ -0,0 +1,86 @@
+#!/usr/bin/env sh
+
+########################################################################
+# Hurricane Electric hook script for acme.sh (dynamic TXT API)
+#
+# These are the pros and cons of dns_he_dyntxt, compared to dns_he:
+# Pros:
+# - No need to store a dns.he.net account password on your server
+# - Uses a very simple write-only API
+# Cons:
+# - You must manually create placeholder _acme-challenge TXT records,
+#   and generate/copy the same DDNS key across all records.
+# - This script WILL FAIL to issue both a domain and its wildcard, because
+#   '-d example.com -d *.example.com' requires multiple TXT records.
+#   Switch to 'dns_he' if you need this feature.
+#
+# Environment variable:
+#   HE_DynTXT_Key - DDNS key for all _acme-challenge TXT records
+########################################################################
+
+# Cheat sheet for passing the DNS.yml API test:
+# - Set TEST_DNS_NO_WILDCARD=1
+# - Create placeholder TXT records for the following domain names:
+#   - _acme-challenge.$TestingDomain
+#   - acmetestXyzRandomName.$TestingDomain
+
+HE_DynTXT_Api="https://dyn.dns.he.net/nic/update"
+
+########  Public functions #####################
+
+#Usage: add  _acme-challenge.www.domain.com   "XKrxpRBosdIKFzxW_CT3KLZNf6q0HG9i01zxXp5CPBs"
+dns_he_dyntxt_add() {
+  fulldomain=$1
+  txtvalue=$2
+
+  HE_DynTXT_Key="${HE_DynTXT_Key:-$(_readaccountconf_mutable HE_DynTXT_Key)}"
+
+  if [ -z "$HE_DynTXT_Key" ]; then
+    HE_DynTXT_Key=""
+    _err "Missing HE_DynTXT_Key. See dnsapi/dns_he_dyntxt.sh for instructions."
+    return 1
+  fi
+
+  #save the DDNS key to the account conf file.
+  _saveaccountconf_mutable HE_DynTXT_Key "$HE_DynTXT_Key"
+
+  _info "Updating record $fulldomain"
+  _he_dyntxt_post "$fulldomain" "$txtvalue"
+  return "$?"
+}
+
+dns_he_dyntxt_rm() {
+  fulldomain=$1
+  txtvalue='""' # The record is just cleared, not removed.
+
+  HE_DynTXT_Key="${HE_DynTXT_Key:-$(_readaccountconf_mutable HE_DynTXT_Key)}"
+
+  _info "Clearing record $fulldomain"
+  _he_dyntxt_post "$fulldomain" "$txtvalue"
+  return "$?"
+}
+
+#####################  Private functions below ##################################
+
+_he_dyntxt_post() {
+  hostname=$1
+  txt=$2
+  response="$(_post "hostname=$hostname&password=$HE_DynTXT_Key&txt=$txt" "$HE_DynTXT_Api")"
+
+  if [ "$?" != "0" ]; then
+    _err "POST failed"
+    return 1
+  fi
+  _debug2 response "$response"
+
+  if _contains "$response" "good" || _contains "$response" "nochg"; then
+    _info "Updated, OK"
+    return 0
+  elif _contains "$response" "badauth"; then
+    _err "'$hostname' missing placeholder TXT record, or DDNS key incorrect"
+    return 1
+  else
+    _err "Unknown POST response: $response"
+    return 1
+  fi
+}