Deduplicate OSConfig.SearchDomains before SetDNS (#12360)

Rather than do this in each implementation, filter the unified
Config.SearchDomains list before calling into the OSConfig layer.

This ensures that when GetBaseConfig rereads that already written
resolv.conf search line, we don't end up growing the same tailnet
entry multiple times.

Updates #12360

Signed-off-by: Kyle McMartin <jkkm@jkkm.org>

net/dns/manager.go

@@ -394,7 +394,18 @@ func (m *Manager) compileConfig(cfg Config) (rcfg resolver.Config, ocfg OSConfig
 			defaultRoutes = append(defaultRoutes, &dnstype.Resolver{Addr: ip.String()})
 		}
 		rcfg.Routes["."] = defaultRoutes
-		ocfg.SearchDomains = append(ocfg.SearchDomains, baseCfg.SearchDomains...)
+
+		seen := map[dnsname.FQDN]bool{}
+		SearchDomains := append(ocfg.SearchDomains, baseCfg.SearchDomains...)
+		// deduplicate our list of SearchDomains
+		ocfg.SearchDomains = []dnsname.FQDN{}
+		for _, sd := range SearchDomains {
+			if seen[sd] {
+				continue
+			}
+			ocfg.SearchDomains = append(ocfg.SearchDomains, sd)
+			seen[sd] = true
+		}
 	}
 
 	return rcfg, ocfg, nil