global: support DNS search domains

This has been supported by Windows and Linux for quite some time. Add
support here for iOS and macOS.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Sources/WireGuardKit/InterfaceConfiguration.swift

@@ -10,6 +10,7 @@ public struct InterfaceConfiguration {
     public var listenPort: UInt16?
     public var mtu: UInt16?
     public var dns = [DNSServer]()
+    public var dnsSearch = [String]()
 
     public init(privateKey: PrivateKey) {
         self.privateKey = privateKey
@@ -25,6 +26,7 @@ extension InterfaceConfiguration: Equatable {
             lhsAddresses == rhsAddresses &&
             lhs.listenPort == rhs.listenPort &&
             lhs.mtu == rhs.mtu &&
-            lhs.dns == rhs.dns
+            lhs.dns == rhs.dns &&
+            lhs.dnsSearch == rhs.dnsSearch
     }
 }

Sources/WireGuardKit/PacketTunnelSettingsGenerator.swift

@@ -85,6 +85,7 @@ class PacketTunnelSettingsGenerator {
 
         let dnsServerStrings = tunnelConfiguration.interface.dns.map { $0.stringRepresentation }
         let dnsSettings = NEDNSSettings(servers: dnsServerStrings)
+        dnsSettings.searchDomains = tunnelConfiguration.interface.dnsSearch
         dnsSettings.matchDomains = [""] // All DNS queries must first go through the tunnel's DNS
         networkSettings.dnsSettings = dnsSettings