RPC calls to LND using curl? I have a bitcoind and LND server running. I can make RPC calls to bitcoind using curl, like this:

curl http://XXX:YYY@localhost:8332 -H 'content-type:text/plain;' --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}'

However I can't find out how to make RPC calls to LND using curl.

My lnd.conf contains a [Bitcoind] section like this:

[Bitcoind]
bitcoind.rpchost=127.0.0.1:8332
bitcoind.rpcuser=XXX
bitcoind.rpcpass=YYY
bitcoind.zmqpubrawblock=127.0.0.1:28332
bitcoind.zmqpubrawtx=127.0.0.1:28333


However, what I'm struggling with is finding out or configuring how to actually connect to LND itself with RPC.

Can't find clear documentation about this. I figured out that apparently I need to use port 10009, but this doesn't work:

curl http://XXX:YYY@localhost:10009 -H 'content-type:text/plain;' --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getinfo","params":[]}'

(result is empty, no error, nothing)

I guessed perhaps I need to set a separate RPC username and password for LND, but I can't find out how or where, i.e. what the settings are in lnd.conf?

Also, do I have to add a config line to make sure the LND daemon is listening to RPC calls? 

Also, do I have to add a config line to make sure the LND daemon is accepting RPC calls from specific IPs? (localhost in this case, or adding external IPs later)
The lnd.conf information you shared above is required for the LND instance to query your local bitcoind node (to detect or follow on-chain/mempool events), and does not represent the LND user interface.

LND has a REST API which you can query with curl:
https://api.lightning.community/rest/index.html#lnd-rest-api-reference 

The REST interface listening port can be configured here:
https://github.com/lightningnetwork/lnd/blob/master/config.go 

The LND documentation describes it in more detail, but you will need to generate valid macaroons for LND https calls.