Merge pull request #4625 from guggero/letsencrypt-listen

lnd+config: allow Let's Encrypt listen IP to be set

sample-lnd.conf

@@ -53,12 +53,13 @@
 ; Sets the directory to store Let's Encrypt certificates within
 ; letsencryptdir=~/.lnd/letsencrypt
 
-; Sets the port on which lnd will listen for Let's Encrypt challenges. Let's 
+; The IP:port on which lnd will listen for Let's Encrypt challenges. Let's
 ; Encrypt will always try to contact on port 80. Often non-root processes are
 ; not allowed to bind to ports lower than 1024. This configuration option allows
 ; a different port to be used, but must be used in combination with port
-; forwarding from port 80.
-; letsencryptport=8080
+; forwarding from port 80. This configuration can also be used to specify
+; another IP address to listen on, for example an IPv6 address.
+; letsencryptlisten=localhost:8080
 
 ; Request a Let's Encrypt certificate for this domain. Note that the certicate
 ; is only requested and stored when the first rpc connection comes in.