Configure IPv6 DNS server using router advertisements.

- Closes #466.
- Adds a `DNSMonitor` that listens for router
  advertisements and updates the resolv.conf
  with the IPv6 DNS server.
- The `configureDns` function now calls
  `DNSMonitor.update()` to write the initial
  configuration.
- The monitor does not block container workload
  startup. IPv6-only containers will need to
  tolerate the initial lack of a DNS server.

Author: jglogan

Package.resolved

@@ -29,6 +29,7 @@ let package = Package(
         .library(name: "ContainerizationEXT4", targets: ["ContainerizationEXT4"]),
         .library(name: "ContainerizationOCI", targets: ["ContainerizationOCI"]),
         .library(name: "ContainerizationNetlink", targets: ["ContainerizationNetlink"]),
+        .library(name: "ContainerizationICMP", targets: ["ContainerizationICMP"]),
         .library(name: "ContainerizationIO", targets: ["ContainerizationIO"]),
         .library(name: "ContainerizationOS", targets: ["ContainerizationOS"]),
         .library(name: "ContainerizationExtras", targets: ["ContainerizationExtras"]),
@@ -205,6 +206,20 @@ let package = Package(
                 .product(name: "Crypto", package: "swift-crypto"),
             ]
         ),
+        .target(
+            name: "ContainerizationICMP",
+            dependencies: [
+                .product(name: "Logging", package: "swift-log"),
+                "ContainerizationExtras",
+            ]
+        ),
+        .testTarget(
+            name: "ContainerizationICMPTests",
+            dependencies: [
+                "ContainerizationExtras",
+                "ContainerizationICMP",
+            ]
+        ),
         .target(
             name: "ContainerizationNetlink",
             dependencies: [