Make device object immutable; support Musl platform (#13)

Author: johnnzhou

Sources/LCLPing/HTTP/HTTPPingClient.swift

@@ -180,7 +180,7 @@ extension HTTPPingClient {
         public let resolvedAddress: SocketAddress
 
         /// The outgoing device associated with the given interface name
-        public var device: NIONetworkDevice?
+        public private(set) var device: NIONetworkDevice?
 
         /// Initialize a HTTP Ping Client `Configuration`.
         ///

Sources/LCLPing/HTTP/NIOHTTPClient.swift

@@ -214,7 +214,7 @@ final class NIOHTTPClient: Pingable {
                     default:
                         ()
                     }
-                    #elseif canImport(Glibc)
+                    #elseif canImport(Glibc) || canImport(Musl)
                     return (channel as! SocketOptionProvider).setBindToDevice(device.name)
                     #endif
                 }

Sources/LCLPing/ICMP/ICMPPingClient.swift

@@ -190,7 +190,7 @@ public final class ICMPPingClient: Pingable {
                     default:
                         ()
                     }
-                    #elseif canImport(Glibc)
+                    #elseif canImport(Glibc) || canImport(Musl)
                     return (channel as! SocketOptionProvider).setBindToDevice(device.name)
                     #endif
                 }
@@ -235,7 +235,7 @@ extension ICMPPingClient {
         public let resolvedAddress: SocketAddress
 
         /// The outgoing device associated with the given interface name
-        public var device: NIONetworkDevice?
+        public private(set) var device: NIONetworkDevice?
 
         public init(endpoint: EndpointTarget,
                     count: Int = 10,

Sources/LCLPing/Utilities/LCLPing+SocketOption.swift

@@ -12,18 +12,26 @@
 
 import Foundation
 import NIOCore
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#elseif canImport(Musl)
+import Musl
+#endif
 
 extension NIOBSDSocket.Option {
     #if canImport(Darwin)
     public static let ip_bound_if: NIOBSDSocket.Option = Self(rawValue: IP_BOUND_IF)
     public static let ipv6_bound_if: NIOBSDSocket.Option = Self(rawValue: IPV6_BOUND_IF)
-    #elseif canImport(Glibc)
+    #elseif canImport(Glibc) || canImport(Musl)
     public static let so_bindtodevice = Self(rawValue: SO_BINDTODEVICE)
     #endif
 }
 
 extension SocketOptionProvider {
-    #if canImport(Glibc)
+    #if canImport(Glibc) || canImport(Musl)
+    
     /// Sets the socket option SO_BINDTODEVICE to `value`.
     ///
     /// - parameters: