multi: add signet parameters

With this commit we make lnd compatible with the public signet test
network.

sample-lnd.conf

@@ -436,6 +436,17 @@ bitcoin.simnet=true
 ; Use Bitcoin's regression test network
 ; bitcoin.regtest=false
 
+; Use Bitcoin's signet test network
+; bitcoin.signet=false
+
+; Connect to a custom signet network defined by this challenge instead of using
+; the global default signet test network -- Can be specified multiple times
+; bitcoin.signetchallenge=
+
+; Specify a seed node for the signet network instead of using the global default
+; signet network seed nodes
+; bitcoin.signetseednode=123.45.67.89
+
 ; Use the btcd back-end
 bitcoin.node=btcd
 
@@ -647,6 +658,10 @@ bitcoin.node=btcd
 ; Use Litecoin's regression test network
 ; litecoin.regtest=false
 
+; Litecoin does not support the signet test network. The options
+; litecoin.signet, litecoin.signetchallenge and litecoin.signetseednode are
+; only defined because the data structure is shared with bitcoind.
+
 ; Use the ltcd back-end.
 litecoin.node=ltcd