Added Selfhost DNS API

+ShellCheck 
+ACME v2 compatible

Example:
- Fist create 2 new TXT records on _acme-challenge.example.com
- Now note the ID in (...) from the edit page behind "_acme-challenge.example.com"

export SELFHOSTDNS_USERNAME=myname
export SELFHOSTDNS_PASSWORD=mypass
export SELFHOSTDNS_RID=id_of_txt_record
export SELFHOSTDNS_RID2=id_of_second_txt_record
acme.sh --issue -d example.com  --dns  dns_selfhost

dnsapi/dns_selfhost.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env sh
+#
+#       Author: Marvin Edeler
+#       Report Bugs here: https://github.com/Marvo2011/acme.sh/issues/1
+
+dns_selfhost_add() {
+  domain=$1
+  txt=$2
+  _info "Calling acme-dns on selfhost"
+  _debug fulldomain "$domain"
+  _debug txtvalue "$txt"
+
+  SELFHOSTDNS_UPDATE_URL="https://selfhost.de/cgi-bin/api.pl"
+  SELFHOSTDNS_USERNAME="${SELFHOSTDNS_USERNAME:-$(_readaccountconf_mutable SELFHOSTDNS_USERNAME)}"
+  SELFHOSTDNS_PASSWORD="${SELFHOSTDNS_PASSWORD:-$(_readaccountconf_mutable SELFHOSTDNS_PASSWORD)}"
+  SELFHOSTDNS_RID="${SELFHOSTDNS_RID:-$(_readaccountconf_mutable SELFHOSTDNS_RID)}"
+  SELFHOSTDNS_RID2="${SELFHOSTDNS_RID2:-$(_readaccountconf_mutable SELFHOSTDNS_RID2)}"
+  SELFHOSTDNS_LAST_SLOT="$(_readaccountconf_mutable SELFHOSTDNS_LAST_SLOT)"
+
+  if test -z "$SELFHOSTDNS_LAST_SLOT"; then
+    SELFHOSTDNS_LAST_SLOT=1
+  fi
+
+  _saveaccountconf_mutable SELFHOSTDNS_USERNAME "$SELFHOSTDNS_USERNAME"
+  _saveaccountconf_mutable SELFHOSTDNS_PASSWORD "$SELFHOSTDNS_PASSWORD"
+  _saveaccountconf_mutable SELFHOSTDNS_RID "$SELFHOSTDNS_RID"
+  _saveaccountconf_mutable SELFHOSTDNS_RID2 "$SELFHOSTDNS_RID2"
+
+  if [ $SELFHOSTDNS_LAST_SLOT = "2" ]; then
+    rid=$SELFHOSTDNS_RID
+    SELFHOSTDNS_LAST_SLOT=1
+  else
+    rid=$SELFHOSTDNS_RID2
+    SELFHOSTDNS_LAST_SLOT=2
+  fi
+
+  _saveaccountconf_mutable SELFHOSTDNS_LAST_SLOT "$SELFHOSTDNS_LAST_SLOT"
+
+  _info "Trying to add $txt on selfhost for rid: $rid"
+
+  data="?username=$SELFHOSTDNS_USERNAME&password=$SELFHOSTDNS_PASSWORD&rid=$rid&content=$txt"
+  response="$(_get "$SELFHOSTDNS_UPDATE_URL$data")"
+
+  if ! echo "$response" | grep "200 OK" >/dev/null; then
+    _err "Invalid response of acme-dns for selfhost"
+    return 1
+  fi
+}
+
+dns_acmedns_rm() {
+  domain=$1
+  txt=$2
+  _debug fulldomain "$domain"
+  _debug txtvalue "$txt"
+}