Switch to using libnetwork's resolvconf package

Libnetwork provides a well-tested package for generating
resolv.conf from the host's that has some features our current
implementation does not. Swap to using their code and remove our
built-in implementation.

Signed-off-by: Matthew Heon <matthew.heon@gmail.com>

vendor/github.com/docker/libnetwork/netutils/utils_linux.go

@@ -0,0 +1,50 @@
+// +build linux
+// Network utility functions.
+
+package netutils
+
+import (
+	"net"
+	"strings"
+
+	"github.com/docker/libnetwork/types"
+	"github.com/vishvananda/netlink"
+)
+
+var (
+	networkGetRoutesFct = netlink.RouteList
+)
+
+// CheckRouteOverlaps checks whether the passed network overlaps with any existing routes
+func CheckRouteOverlaps(toCheck *net.IPNet) error {
+	networks, err := networkGetRoutesFct(nil, netlink.FAMILY_V4)
+	if err != nil {
+		return err
+	}
+
+	for _, network := range networks {
+		if network.Dst != nil && NetworkOverlaps(toCheck, network.Dst) {
+			return ErrNetworkOverlaps
+		}
+	}
+	return nil
+}
+
+// GenerateIfaceName returns an interface name using the passed in
+// prefix and the length of random bytes. The api ensures that the
+// there are is no interface which exists with that name.
+func GenerateIfaceName(prefix string, len int) (string, error) {
+	for i := 0; i < 3; i++ {
+		name, err := GenerateRandomName(prefix, len)
+		if err != nil {
+			continue
+		}
+		if _, err := netlink.LinkByName(name); err != nil {
+			if strings.Contains(err.Error(), "not found") {
+				return name, nil
+			}
+			return "", err
+		}
+	}
+	return "", types.InternalErrorf("could not generate interface name")
+}