DNS prefs: use thread-safe DNS settings getter

__res_state can't be used with pthreads involved, instead use
__res_get_state and __res_put_state which maintain a pool of res_state
instances.

Fixes #12195.

src/add-ons/network_settings/dnsclient/DNSSettingsView.cpp

@@ -196,7 +196,7 @@ DNSSettingsView::_LoadDNSConfiguration()
 	if (res_init() != 0)
 		return B_ERROR;
 
-	res_state state = __res_state();
+	res_state state = __res_get_state();
 
 	if (state != NULL) {
 		for (int i = 0; i < state->nscount; i++) {
@@ -206,6 +206,8 @@ DNSSettingsView::_LoadDNSConfiguration()
 		}
 
 		fDomain->SetText(state->dnsrch[0]);
+
+		__res_put_state(state);
 		return B_OK;
 	}