ui,tunnel: support DNS search domains

wg-quick has supported this for a while, but not the config layer, and
not the Go backend, so wire this all up.

Requested-by: Alexis Geoffrey <alexis.geoffrey97@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tunnel/src/main/java/com/wireguard/android/backend/GoBackend.java

@@ -274,6 +274,9 @@ public final class GoBackend implements Backend {
             for (final InetAddress addr : config.getInterface().getDnsServers())
                 builder.addDnsServer(addr.getHostAddress());
 
+            for (final String dnsSearchDomain : config.getInterface().getDnsSearchDomains())
+                builder.addSearchDomain(dnsSearchDomain);
+
             boolean sawDefaultRoute = false;
             for (final Peer peer : config.getPeers()) {
                 for (final InetNetwork addr : peer.getAllowedIps()) {