lncfg: add isIPv6Host helper to force v6 addrs through system resolver
lncfg: add isIPv6Host helper to force v6 addrs through system resolver

With this commit, if --tor.active is specified, then IPv6 addresses
will no longer go through the connmgr.TorLookupIP function from btcd.
This function does not have proper IPv6 support and would fail with
the error "tor general error". Instead, use the system resolver.

aakselrod/lnd,aakselrod/lnd