net/dns: split out search domains and match domains in OSConfig.

It seems that all the setups that support split DNS understand
this distinction, and it's an important one when translating
high-level configuration.

Part of #953.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/manager.go

@@ -66,8 +66,7 @@ func (m *Manager) Set(cfg Config) error {
 		Routes:       map[string][]netaddr.IPPort{},
 	}
 	osCfg := OSConfig{
-		Domains: cfg.SearchDomains,
-		Primary: true,
+		SearchDomains: cfg.SearchDomains,
 	}
 	// We must proxy through quad-100 if MagicDNS hosts are in
 	// use, or there are any per-domain routes.