feat: allow network interface to be ignored

Added a property to userdata to allow a network interface to be ignored,
such that Talos will perform no operations on it (including DHCP).

Also added kernel commandline parameter (talos.network.interface.ignore)
to specify a network interface should be ignored.

Also allows chaining of kernel cmdline parameter Contains() where the
parameter in question does not exist.

Fixes #1124

Signed-off-by: Seán C McCord <ulexus@gmail.com>

internal/pkg/kernel/kernel.go

@@ -82,6 +82,10 @@ func (v *Parameter) First() *string {
 
 // Get attempts to get a string from a value's internal representation.
 func (v *Parameter) Get(idx int) *string {
+	if v == nil {
+		return nil
+	}
+
 	if len(v.values) > idx {
 		return &v.values[idx]
 	}
@@ -91,6 +95,10 @@ func (v *Parameter) Get(idx int) *string {
 
 // Contains returns a boolean indicating the existence of a value.
 func (v *Parameter) Contains(s string) (ok bool) {
+	if v == nil {
+		return ok
+	}
+
 	for _, value := range v.values {
 		if ok = s == value; ok {
 			return ok