mxrouterctl: Add veth feature

This adds a `veth(NAME)` config call which creates a pair of connected
veth devices, one in the default namespace and the other one on the
router namespace, both with the same name.

This can be used, for example, to create interfaces on the host for VMs
with the traffic routed through a mxrouter instance.

Example:

interface('vlan.irouter');
ip('vlan.irouter','172.19.141.2/24');
route('default','172.19.141.1');

veth('veth.variation');

my $NET_VARIATION = '172.19.118.0/24';
ip('veth.variation', '172.19.118.1/24');

my $NET_WISNET='141.14.16.0/20';

rule('filter','FORWARD',"-m state --state ESTABLISHED,RELATED -j ACCEPT");
rule('filter','FORWARD','-p icmp --icmp-type 8 -j ACCEPT');                  # ping

rule('filter','FORWARD',"-d $NET_VARIATION -j variation-out");
rule('filter','variation-out',"-s $NET_WISNET -p tcp --dport 22 -j ACCEPT"); # ssh
rule('filter','variation-out',"-j DROP");

rule('filter','FORWARD',"-j ACCEPT");

mxrouter/mxrouterctl

@@ -374,6 +374,7 @@ sub radvd {
 }
 
 our $want_if;
+our $want_veth;
 
 our $DHCRELAY_FORWARD;
 our %DHCRELAY_IF;
@@ -483,7 +484,7 @@ sub start {
 
 	netif_is_up('lo') or sys('ip link set lo up');
 
-	for my $dev (keys %$want_if) {
+	for my $dev (keys %$want_if, keys %$want_veth) {
 		netif_is_up($dev) or sys('ip','link','set',$dev,'up');
 	}
 
@@ -622,6 +623,11 @@ sub interface {
 	$want_if->{$dev}=1;
 }
 
+sub veth {
+	my ($dev) = @_;
+	$want_veth->{$dev} = 1;
+}
+
 my @SAVED_ARGV=@ARGV;
 
 GetOptions(OPTIONS) or die USAGE;
@@ -776,6 +782,10 @@ if (!$opt_this_ns) {
 		for my $dev (sort keys %$want_if) {
 			move_dev_into_ns($dev,$NETNS);
 		}
+		for my $dev (sort keys %$want_veth) {
+			sys('ip', 'link', 'add', $dev, 'type', 'veth', 'peer', 'name', $dev, 'netns', $NETNS);
+			sys('ip', 'link', 'set', $dev, 'up');
+		}
 		system('ip','netns','exec',$NETNS,$0,'--this-ns',@SAVED_ARGV) and exit 1;
 	} elsif ($cmd eq 'stop') {
 		have_netns($NETNS) or die "not running (network namespace $NETNS does not exist)\n";