lnd: expose user agent version as config option

Expose the neutrino `UserAgentVersion` config option. This can be set
by starting lnd with the `--neutrino.useragentversion=` flag.

docs/INSTALL.md

@@ -198,6 +198,7 @@ neutrino:
       --neutrino.banduration=                                 How long to ban misbehaving peers.  Valid time units are {s, m, h}.  Minimum 1 second
       --neutrino.banthreshold=                                Maximum allowed ban score before disconnecting and banning misbehaving peers.
       --neutrino.useragentname=                               Used to help identify ourselves to other bitcoin peers.
+      --neutrino.useragentversion=                            Used to help identify ourselves to other bitcoin peers.
 ```
 
 ## Bitcoind Options